In the post-bipolar era, the security architecture of Europe had to be adapted and the main European security actors undertook internal and external changes. The principal challenge has been the re-approximation of former enemies, namely the North Atlantic Treaty Organization (NATO) and Russia. The main remaining institution available to assume security and defense responsibilities was NATO. Original institutionalized frameworks of cooperation have, then, been created to deal with Moscow that is not willing to be a member. Additionally, the Kremlin has moved towards its own institutional arrangements, such as the creation of the Collective Security Treaty Organization (CSTO).

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