During the Cold War, NATO’s role and purpose were clearly defined by the existence of the threat posed by the Soviet Union. By the early 1990s, the Warsaw Pact had been dissolved at the insistence of the newly liberated countries of Eastern Europe, and the Soviet Union had collapsed. With the disappearance of traditional adversaries, some commentators believed that the need for NATO had also been removed and that future defence expenditure and investment in armed forces should be dramatically reduced. It soon became apparent that although the end of the Cold War might have removed direct military threats, instability in some parts of Europe had increased. A number of regional conflicts, often fueled by ethnic and religious tension have increased the necessity of an NATO. On the other hand, while the threat to the West after the Cold War reduced, however, regional conflicts in the 1990s was not prevented by international security and political organizations such as the UN, OSCE or the EU. This situation increased the need for NATO as it intervened in major conflicts in Balkans, Afghanistan and in Somali, Libya etc. NATO assumed important functions as a security organization. NATO has played a decisive role in consolidating democracy in Central and Eastern Europe, in the protection of human rights in the Balkans and in political development. In the aftermath of the 9/11 attack, it began to perform of out of are missions, particularly in Afghanistan, Libya, the Mediterranean Sea, and offshore in the Gulf of Aden and Somalia. NATO has also undertaken new tasks such as energy security, counter-terrorism, cyber defense and counter-bandit. Together with these tasks, NATO is contributing to strengthening the military capacities of non-member states in Eurasia, North Africa and the Middle East through partnership programs. Finally, NATO is currently a non-combatant training and support mission in Afghanistan, peacekeeping operations in Kosovo, monitoring operations in the Mediterranean, peacekeeping missions in support of the African Union in Africa, air policing missions in Ukraine and Air Policing in Africa. In short, it can be said that NATO, which played an important role in the end of the Cold War, played an important role in the establishment of global security in the 1990s and 2000s. NATO has also been a charming venue for newly established countries by offering global security umbrellas to its member through collective defence understanding. These developments paved the way for NATO to become a global power. However, after resurgent Russia as assertive global politic power in 2010s, NATO has revisited its security perceptions. NATO's new security threats are more concrete, visible and evident. The threats are conceptualized to reflect conventional (Russian Federation), asymmetric (terror) and hybrid (cyber) kinds. NATO is now restructuring itself to compete with new threats that are more visible than in previous periods. As a result, the phenomenon of coexistence, which includes peace and cooperation in the international environment after 1990, has begun to disappear. The state of insecurity has now become prevalent in interstates relations. NATO”s brilliant years have almost disappeared as it has faced several challenging issues. First, American president Trump has very harsh critiques to NATO, during his election campaign. He sees NATO as an organization consuming American wealth. He criticized the other members not sharing of economic burden of NATO equally. So, America seems less supporter of NATO comparing with past although Republicans are in power. America wants to hand over European security issues to Europeans as it has shifted its security priorities from Europe to Asia. Second, global economic crisis has negatively affected NATO members in increasing their military spending. Although NATO took several decision to increase military spending until 2% of respective GDPs, except a few members, this target was not attained. Thus, less spending means the weakening of NATO in financial capacity. Third, some European members want to establish a separate European security architecture free from NATO and The USA. They want to get rid of dependency to America in security and military perspective. This is another factor leading to decline of NATO. Fourth, Russia exploits the opinion differences among NATO members to weaken NATO alliances by utilizing its energy capacity, arms sailing and other charming methods. The last but not least, the conflicts and crisis in Middle East has led to some divergence among NATO members. Finally, as the Western economically declines, the Western military is also declining. Considering all these explanation NATO seems in declining process. However, although current political, economic and military situation have created an unfavorable condition for NATO, it is too early to arrive has successfully overcome many challenges in the past, it can also resolve the current challenges. [ABSTRACT FROM AUTHOR]