Sulym B.
Associate Professor, Candidate of Political Sciences,
Docent of Department International Relations and Diplomacy,
Ivan Franko National University of Lviv

Abstract
Aggression of Russian against Ukraine in 2014 became a serious challenge for European security. It demonstrated the need for a strong and effective security architecture on NATO’s eastern flank, where Ukraine plays a key role. This article will examine Ukraine’s role in ensuring the security of NATO’s eastern flank. In particular, the article considers the following issues:

• The geopolitical importance of Ukraine at the crossroads of Europe and Russia, as an important aspect of international cooperation.
• Ukraine’s military capabilities due to access to military forces and the ability to make a significant contribution to the defense of NATO’s eastern flank.
• Reforms in the security and defense sector of Ukraine in order to increase its compatibility with NATO.
• Ukraine’s cooperation with NATO within the framework of various programs aimed at increasing its defense capabilities.

The balance of power in the world is constantly changing, so it is important to take into account current threats and challenges when forming a security architecture. Each country has its own national interests that must be taken into account when developing a common security architecture. No country can guarantee its security alone, so it is necessary to develop cooperation with other countries and international organizations.
The paper examines NATO’s main functions in ensuring cross-border security: guaranteeing collective defense and deterring potential aggressors; participation in peacekeeping operations and security capacity-building programs in various regions of the world; supporting the development of democracy and the rule of law for its members and partner countries; active fight against international terrorism through joint military operations and other measures and others.
The tools of NATO for the formation of a security space were considered: powerful military forces used to deter aggression and protect the members of the Alliance; regular political consultations among its members on security and defense issues; cooperation with other countries and international organizations in order to strengthen international security.
The main principles of building a security architecture:

1. The security of one country cannot be guaranteed at the expense of the security of another.
2. Security architecture should be able to deter potential aggressors.
3. The security architecture should be transparent and predictable for all participants.
4. The security architecture must be able to adapt to changes in the geopolitical situation and the emergence of new threats.

Keywords: Ukraine, NATO, eastern flank, security, geopolitics, military capabilities, reforms, cooperation