NATO has more than doubled in size over its lifetime, expanding from a core group of 12 countries to its current 28 members. There have been six main phases of expansion, three occurring during the Cold War and three after. Turkey and Greece were the first new members, brought into the alliance in 1952 to cement the prevailing NATO strategy of containment. West Germany's integration in 1955 contributed to the formation of the Warsaw Pact, a Soviet-led counterbalance to NATO that incorporated eight communist states across Central and Eastern Europe. Spain's 1982 induction into the alliance helped further consolidate Western Europe. The final three stages of NATO's expansion occurred in 1999, 2004 and 2009, bringing former communist countries into play and adding considerable depth to NATO's northern sector.
This growth enabled the organization to develop a much more flexible and responsive strategy than the previous forward defense concept allowed. The move toward a more versatile NATO structure, cutbacks in defense spending and the perception of Russia as less dangerous factored heavily into the development of a rotational deployment framework for NATO members. The massive, permanent concentrations of force that typified the Cold War slowly began to dissolve.
The expansion of NATO was not without its consequences. Because the alliance requires unanimous consent to approve any action, increased membership also raises the risk of divergent interests, heightening the possibility of a NATO constituent vetoing a proposal. By getting so close to Russia's borders, NATO succeeded in agitating Moscow, which views the proximity of the alliance as a genuine threat. As a result, Russia has worked continuously to undermine any Western neighbor.
A resurgent Russia is enough to make Eastern Europe nervous -- even the NATO members. If anything, their perception of the threat is greater because they are closest to Russia and have the least overall protection. Potential threats are varied and numerous, from airspace incursions by Russian aircraft, to the deployment of theater ballistic missiles in nearby locales, to cultural subversion and economic coercion. Western Europe, in comparison, enjoys strategic depth and greater economic security while at the same time possessing the bulk of the remaining NATO infrastructure and combat power.