The North Atlantic Treaty Organization (NATO) addresses the common security concerns of its 26 member countries from North America and Europe. Created in 1949, to combat the spread of communism in Europe, NATO has a defensive purpose. All decisions are made by consensus, meaning all countries must unanimously agree before NATO takes action. After the collapse of the Soviet Union, NATO faced a bit of an identity crisis.
NATO's mission in Afghanistan marked the first time ever NATO took action outside of the Euro-Atlantic area. There are currently 50,000 NATO troops in Afghanistan, working toward security and stability. NATO had no part in the Iraq invasion, but since August 2004, it has been active in training Iraqi security forces. In Kosovo, NATO leads 15,000 international forces. Its UN-mandated presences, intended to maintain Kosovo's security, will be tested by Kosovo's recent declaration of independence and the Serbian response.
Since the end of the Cold War, former communist countries, as well as ex-republics of the Soviet Union have joined NATO. The swallowing of countries formerly under Russia's influence troubles Putin and may cause Russia to obstruct US policy going forward. Russia, a permanent member of the UN Security council, may make it difficult for the US to do what it wants with Iran, for example.
For some useful sources on NATO,