Key Events of the Second Period of the Armed Conflict in the East of Ukraine (05. 09. 2014 – 30. 04. 2018)

The article aims to structure the accumulated factual material about the Russian Federation's armed aggression against Ukraine, which began on 20 February 2014 and developed a historical periodization variant. The research's main result is that the authors prove that the Russian Federation's armed aggression caused the armed conflict in the East of Ukraine. The authors set out the actual course of the main events in chronological sequence and determined the Russian-Ukrainian armed confrontation's key milestones at the beginning of the 21st century. According to the nature of military operations, military-political results and the consequences of the armed conflict in the East of Ukraine, it is divided into periods within which stages are distinguished, qualitatively different in purpose, nature and content of combat (special) operations. To repulse and deter this aggression, Ukraine used the forces of the security and defence sector. The form of these forces employment from 14 April 2014 to 30 April 2018 was the Anti-Terrorist Operation, and after – the Joint Forces Operation. This military-historical study creates the basis for further scientific research on the study of the armed conflict in the East of Ukraine. Such research is necessary to have a clearer understanding of the nature and essence of Russia's hybrid war against Ukraine, a more in-depth analysis of its evolution, and broader dissemination of lessons learned in the course of countering hybrid threats in the military sphere.


INTRODUCTION
The Russian Federation's armed aggression against Ukraine that began on 20 February 2014 and had a hybrid nature led to the occupation of Crimea and caused an armed conflict in the East of Ukraine. In the second issue of the "Codrul Cosminului" for 2018, the authors highlighted the first period of the armed conflict in the East of Ukraine. 1 Then we promised to continue our study.
The article aims to offer the scientific community a variant of the armed conflict periodisation in the East of Ukraine, structure the accumulated factual material and create a basis for further scientific research on this phenomenon's study. It helps researchers build a clear understanding of the nature and essence of Russia's hybrid war against Ukraine, analyse its evolution, and generalise the lessons learned in the course of countering hybrid war in the military sphere.
A military historical analysis of Russian armed aggression anticipated a sequence of successive research stages. 2 The knowledge building on a historical process begins with a chronological fixation of past events and their description.
However, the chronology itself is purely descriptive because the task of carrying out a meaningful analysis of the considered phenomena and processes is not assumed at that stage. The chronologic description captures the events dispassionately in the flow of time, and it does not establish causal relationships and does not provide an assessment of historical trends and patterns. Nevertheless, relying on a reliable and complete historical chronicle of events related to the aggression of the Russian Federation, and applying a scientific methodology, one can not only find cause-effect relationships but also reveal the objective laws which stand behind these relationships and determine that they acquire new stable qualities, becoming the objective laws themselves.

ARGUMENTATION
Military historical science considers periodisation as one of the methods for studying military conflicts. It allows dividing wars and armed conflicts into specific time extents (periods, phases, stages), which qualitatively differ in military-political objectives, the nature and content of military operations. The boundaries of the period are determined by events that mark the turning points in military conflicts. Each period covers a certain number of military campaigns and operations. 3 It is important to note that military operations are planned and conducted to achieve designed objectives. In turn, during military-historical research, the processes describing past events are usually considered in stages. 4 In Ukrainian military-historical literature, the terms "phase" and "stage" are actively used for the historical processes' periodisation, in particular, to denote the substructural components of a period. 5 Historians use different types of periodisation of military conflicts, especially, to distinguish between strategic and historical periodisation. Strategic pe- riodisation is established during the planning of the war and is implemented during its development. This involves the breakdown of the war into several successive parts (campaigns), each solving specific problems through the employment of a particular array of military forces. Simultaneously, military historians use periodisation, which divides the war by actual progress according to the results achieved. Historical periodisation is determined by the sequence of actions, each of which has its name following its characteristic content. 6 The periodisation of the Russian Federation's armed aggression against Ukraine, the armed conflict in the East of Ukraine and the anti-terrorist operation/the Joint Forces Operation have already become a subject to some studies 7 , diversified in terms of issues under consideration and form.
In the project developed by "Information Resistance" group and titled The Invasion of Ukraine. The Chronicle of Russian Aggression, the armed conflict events are presented through the authors' analysis of Russian aggression in 2014-2016, which is conditionally divided into 11 stages. Because the book appeals to a broad audience, the armed conflict periodisation is more popular than scientific. 8 In his scientific work The War in the East of Ukraine: the first phase (1 March -24 August 2014), Pavlo Guy-Nyzhnyk, a Ukrainian historian, divides the events of the Russian-Ukrainian armed conflict into five periods. Giving the names for the periods, the author uses the style of large-scale periodisation of big wars ("position war", "frail war"). Still, instead of using military terminology, he fails to avoid the use of literature cliches (like "Front without resistance", "Minsk trap", "Norman odds", etc.  10 The representative of Lviv Military History School, Doctor of Historical Sciences, Olesia Kutska divides the anti-terrorist operation into two periods within which the researcher allocates 14 stages "to identify and isolate indirect changes in studying the experience of using the Armed Forces and other paramilitary formations of Ukraine (emphasised by us because the term is not precise) within the framework of the anti-terrorist operation in 2014 -2018." 11 However, the researcher does not explain why there is the gap between the second (21. 05 -06. 07. 2014) and third (07. 07. 2014) stages of the manoeuvring period (emphasised by us), and does not take into account that the beginning of active actions of the anti-terrorist operation forces accounts for 1 July 2014 -following the deci- Since early September 2014, at the fourth stage of the anti-terrorist operation in the Donetsk and Luhansk oblasts, ATO forces were to move onto the defensive due to Russian troops' invasion. This stage's characteristic feature was the beginning of large-scale construction of the defence barrier in the East of Ukraine, which started in September 2014. Two defence lines were created: the first oneon the confrontation line with the Russian occupation troops, and the second one -at 15-20 km from it. Their basis was a system of strongholds with checkpoints, trenches, shelters, communication trenches, and firing positions of armoured combat vehicles and other firearms. 15 As of the end of 2014, ATO forces performed the tasks of isolating the crisis area by covering positions and conducting defence at 94 checkpoints, three observation posts, 110 platoon strongpoints, 30 company strongpoints, and one battalion defence area.
Russian hybrid warfare occupation forces continuously committed acts of provocation aimed at stirring up the armed conflict. 16 Thus, from 5 September 2014 to 15 January 2015, Ukrainian troops were shelled 21 250 times. 17 Due to the enemy forces' insidiousness and the negligence of the Minsk agreements by the Russian leadership during the ceasefire, the armed confrontation in the East of Ukraine acquired the features of the protracted 'frozen' conflict. The so-called 'regular armed forces' of the 'DPR' and 'LPR' ('1st and 2nd Army Corps') were formed from the various illegal armed groups of Russian mercenaries and local collaborators, under the leadership of Russian cadre officers (see Figure No 1). The share of the local population in the mentioned units did not exceed 35%. These forces were controlled and supported from the Russian Federation territory. 18 The winter of 2015 proved to be no less 'hot' than the summer of 2014. In January, the enemy began to implement a plan to expand the controlled territories, which involved simultaneous offensives in the Luhansk, Donetsk, Debaltseve, and Mariupol directions.
One of the 'hottest' places in the ATO was the Donetsk airport, which became a symbol of courage and heroism of Ukrainian fighters, whom the enemy called 'Ukrainian cyborgs'. It became a kind of 'pain in the neck' for Russian invaders. It is not a coincidence that elite Special Forces units of the Russian Armed Forces were involved in the seizure of this objective. The 242-day defence of the airport involved not only holding of the terminal buildings and the control tower. The fighting also continued for the control of Avdiivka, Pisky, Opytne, and Vodiane localities. The Ukrainian units' sustained defence at this sector of the confrontation line between the warring parties provided a reliable cover for the whole Donetsk direction. 20 In the history of victories of the Ukrainian army, the days of January 2015 are filled with tragedy and sacrifice of Ukrainian soldiers. On 15 January, the enemy began the assault with artillery and mortar fire at the new Donetsk airport terminal. Exploiting fires' effects, the enemy assumed the offensive, seized part of the airport and blocked the Ukrainian paratrooper units in the 'new terminal'. Since the Russians blew up the terminal, the construction's main elements collapsed, burying its defenders. On 22 January, the last small group of Ukrainian fighters left the terminal ruins in which there were no longer any walls that could be defended. 21 Late in January, Russia's hybrid occupation forces launched an offensive on the Debaltsevo bridgehead, trying to surround and destroy the Ukrainian troops located there. For this purpose, the enemy concentrated significant forces of the Russian Federation's regular troops and the most combat-ready illegal armed formations of self-proclaimed "republics" (see Figure No. 2). The strength of enemy troops reached 7-9 thousand people, supported by up to 120 tanks, 180 artillery systems, 60 multiple launch rocket systems. 22 Following the results of lengthy negotiations on 15 February 2015, a joint declaration was adopted by the Presidents of Ukraine, the French Republic, the Russian Federation and the Chancellor of the Federal Republic of Germany to support the package of measures for the legal implementation of the Minsk agreements. According to this document, the ceasefire regime was to come into effect, and from 24 February the withdrawal of heavy weapons from the contact line was to begin. 26 However, the aggressor insidiously violated these agreements, like the previous ones. In 2015 alone, starting on 15 February, ATO forces were fired at more than 16,4 thousand times; 205 armed clashes took place. The Ukrainian units opened fire in response to the enemy's provocative actions mainly during armed clashes, providing the information to the leadership of the OSCE Special Monitoring Mission accordingly. 27 24 Ibid. 25

The sixth stage. Strengthening the Defence Line in Eastern Ukraine (21 February 2015 -20 September 2016)
In order to provide the militaries with favourable conditions for conducting defence operations, the first task was to establish a line of defence in the East of Ukraine, equipped with powerful fortifications. The efforts and resources from all regions of Ukraine were involved in this task.
After the second offensive of the Russian occupation forces was repulsed in January -February 2015, the construction of a fortified defence line in the eastern regions has become the priority goal of Ukraine. In the spring and summer of 2015, 20 regions of Ukraine contributed to the construction of a fortified line of defence in the ATO area. The total volume of building materials used in the construction course was 35 thousand railway cars, 12 thousand concrete structures were installed in the ATO area. Three defensive lines were erected, at which about 300 strongpoints were equipped and connected by 600 km of fortifications. 28 The Armed Forces units and detachments occupied the first line of defence, and the objects of the second and the third lines were taken under the protection of the National Guard units.
The construction and improvement of the defence line in Eastern Ukraine continued throughout 2015 and after. Due to the construction of protective structures, the number of military casualties was reduced.
Thus, through joint efforts of the grouping of forces and means of the anti-terrorist operation, the authorities of state executive power and local self-government, fortified defence belts were built.
The construction of the defence line in the East of Ukraine became a significant step towards improving the Ukrainian territory's protection system. Consolidated fortifications increase personnel's security and create better conditions for military units and detachments to perform combat missions. The experience of warfare has shown the utmost importance of the fortification equipment of checkpoints, strongholds, base camps and control points to ensure the maintenance of life of personnel and skilful cover by engineering barriers to prevent sudden attacks by illegal armed groups.
Simultaneously with the creation of the defensive zone, a high level of professionalism was acquired by Ukrainian servicemen. In the second anti-terrorist operation period (05.09.2014 -30.04.2018), the sector's forces' primary efforts were focused on stabilising the confrontation line, repelling attacks by Russian occupying troops, preventing the displacement of anti-terrorist operation forces from occupied strongholds and bypassing them. Defence actions of tactical groupings were planned and carried out.
A demonstrative example of troops' employment in military operations was the defensive battle of the 2nd Battalion Tactical Group of the 28th Separate Mechanized Brigade near Marinka in summer 2015. This locality in the outskirts of Donetsk was always a fighting ground. On 3 June, intensive artillery and "Grad" MLRS salvos were fired at ATO forces' positions. The enemy tried to break through the Ukrainian defences in a narrow area, where considerable infantry and armour forces were sent. Tank units supported the assault. To avoid devastating fire, Ukrainian defenders had to abandon some positions. ATO forces used artillery units to repel the enemy's offensive and thus the opponent's advance was stopped. (see Figure No. 3). The enemy retreated towards Donetsk. 29 Since 2015, a system of military-civilian administrations was established and operated in the area of anti-terrorist operation as temporary state authorities in Donetsk and Luhansk regions' territory. They became part of the Anti-Terrorist Centre under the Security Service of Ukraine.
A civil-military cooperation system was introduced in the Armed Forces of Ukraine to prevent social tension in the area of anti-terrorist operation. The civilian population was supported by military and non-military personnel and assets in addressing their vital issues. These activities made it possible to minimise the effects of hostilities on civilians and enhance the ability to counter the negative Russian information influence on the population in the state.
In the face of significant threats to national security, Ukraine was forced to take several measures to localise the conflict in some districts of Donetsk and Luhansk regions. In particular, in cooperation with other entities of the nationwide counter-terrorism system, the Security Service of Ukraine introduced and monitored the movement of persons, vehicles and goods across the confrontation line within the Donetsk and Luhansk regions.
In 2016-2018, the Russian Federation's hybrid war against Ukraine continued, and the force component remained an essential means of achieving its political goals. The Russian Federation's leadership sought to form a militant group in the occupied territory of Donbas that would pose a significant military threat to Ukraine in the eastern region. Numerous facts of issuing Russian passports to Ukrainian citizens living in the occupied territories reinforce Russian aggression's hybrid nature. 30 Taking into account the forces and means of the so-called "1st and 2nd Army Corps", Russian occupation troops in the Donbas in the first half of 2016 reached the strength of almost 43 000 pax, more than 700 tanks and 1 330 combat armoured vehicles, nearly 750 pieces of artillery systems. 32 In the context of a particular limitation of hostilities, Russia intensified the activities of special services, undercover agents, terrorist and sabotage groups in some areas of Donetsk and Lugansk regions, and the adjacent areas of Ukraine. It increased the number of outreach and other activities aimed at destabilising the East of Ukraine's situation.
The forms and techniques of employment of Russian hybrid occupation forces in 2016-2018 changed significantly compared to 2014-2015. Having abandoned active offensive actions, the Russian troupes in the Donbas switched to tactics of "harassing" activities, the purpose of which was to exhaust and demoralise the ATO forces. The enemy tried to provoke the soldiers of the Armed Forces of Ukraine to an active response in order to accuse Ukraine of violating the Minsk agreements. As part of this tactic, the enemy typically resorted to shelling ATO forces, laying mines on roads and bridges and launching armed attacks by sabotage and reconnaissance groups. 33 The seventh stage. Separation of forces and military equipment of warring parties (21 September 2016 -November 2017).
Since September 2016, the nature of the Russian-Ukrainian confrontation in the East of Ukraine changed significantly, mainly due to the Tripartite Contact Group decision, and to the signing on 21 September 2016 in Minsk on the separation of forces and assets of belligerent parties in the Donbas. The document provided for the withdrawal of troops by both sides from the positions they had occupied in both directions for the formation of 2 km wide and 2 km deep sections. 34 The Russian occupation forces continued to violate the agreement, did not  remained one of the hottest spots in the armed conflict zone. 36 Thus, the seventh stage is characterised by the separation of forces and equipment of the warring parties, in accordance with the results of the Tripartite Contact Group decision. Simultaneously, throughout the perimeter of the armed conflict, the enemy was concentrating its efforts on advancing its units deep into the "grey zone". The most intense fighting took place in Svitlodar Arc (December 2016). The Ukrainian forces repelled the Russian occupation forces, exploited success and improved their tactical situation in the Avdiivka industrial zone (December 2016 -January 2017). 37 At that time, the troops' main efforts were focused on stabilising the line of contact between the parts, repelling the attacks of the Russian occupation troops and preventing the displacement of anti-terrorist operation forces from occupied strong points and bypassing them. The defensive operations of tactical groups were planned and carried out.
The eighth stage. The improvement of troops command and control system.

Completion of the anti-terrorist operation (November 2017 -April 2018).
It should be noted that Ukraine's confrontation with Russian aggression in 2014-2017 showed that the anti-terrorist operation, as a form of opposition to attack, had already gone beyond its intended purpose and had a military-defensive character. At the end of 2017 -beginning of 2018, the current challenge was to update the legal framework of using defence forces to protect the state from the Russian Federation's armed aggression.
On 24 February 2018, the Law of Ukraine "On Features of State Policy for ensuring state sovereignty of Ukraine in temporarily occupied territories in Donetsk and Luhansk regions" came into force and changed the previous format of the anti-terrorist operation and the procedure of using the Armed Forces of Ukraine, other military formations and law enforcement authorities. Therefore, the legal framework was created to launch the Joint Forces operation to ensure national security and defence, to repel and deter armed aggression in the Donetsk and Luhansk regions. 38 On 30 April 2018, the President of Ukraine signed a decree "On approval of the decision of the Council of National Security and Defence On a large-scale antiterrorist operation in Donetsk and Luhansk regions". The same day, he (the President of Ukraine), as Supreme Commander-in-Chief of the Armed Forces of Ukraine, signed the order "On the commencement of the Joint Forces operation to ensure national security and defence, to repel and deter the armed aggression of the Russian Federation in Donetsk and Luhansk regions". According to the order from 2:00 p.m. on 30 April 2018, the Joint Force Operation began. 39 Therefore, the second period of the armed conflict in the East of Ukraine can be regarded as a positional one, that is characterised by the transition to predominantly defensive operations, the requirements for the withdrawal of heavy weapons by both sides from the delimitation line and the creation of security zones; stable holding of designated areas, boundaries and positions by the anti-terrorist operation forces; slow advancing of our units into the depth of the territory and taking under the control of certain localities of the so-called "grey zone"; constant violations of the Minsk Agreements by the Russian side, shelling of positions of Ukrainian troops by the Russian occupation forces and its adherence to the tactics of "war on exhaustion".
The proposed periodisation of the armed conflict in the East of Ukraine has been tested on international and all-Ukrainian scientific conferences 40 .

CONCLUSIONS
The Russian Federation's armed aggression against Ukraine and occupation of the part of her territory testified that the application of the forces was mainly a departure from the traditional forms and methods of warfare and had elements of a hybrid nature. The Russian Federation's armed aggression initially led to the occupation of the Crimea and subsequently to an armed conflict in the East of Ukraine. To counteract and deter this aggression, Ukraine has deployed the security and defence sector forces. The form of employment of the security and defence forces from the 14 April 2014 was the Anti-Terrorist Operation, and from 30 April 2018, it is the Joint Forces Operation. In our opinion, the "Donbas War" is rightly classified as an armed conflict in the East of Ukraine, supported externally.
By the nature of the warfare, military and political results and consequences, the armed conflict in Eastern Ukraine is divided into three periods. Within these periods, the stages are distinguished qualitatively in the nature and content of the warfare.
Along with this, many aspects of the ATO still need to be thoroughly researched by military scientists and representatives of various fields of knowledge. In particular, the conceptual categorical apparatus associated with the Russian Federation's aggression against Ukraine, given its specific "hybrid" nature, still needs further clarification and improvement in the scientific works of researchers of this problem. In general, a comprehensive study of the armed conflict in the East of Ukraine is necessary to have a clear understanding of the nature of Russia's hybrid war against Ukraine, to analyse the evolution of the conflict and, ultimately, to summarise the experience of counteracting hybrid threats in the military sphere. The