North Atlantic Treaty Organization

After the fall of the Berlin Wall in 1989, the organization became drawn into the Balkans while building better links with former potential enemies to the east, which culminated with several former Warsaw Pact states joining the alliance in 1999 and 2004. On 1 April 2009, membership was enlarged to 28 with the entrance of Albania and Croatia. Since the 11 September attacks, NATO has attempted to refocus itself to new challenges and has deployed troops to Afghanistan as well as trainers to Iraq.

The Treaty of Brussels, signed on 17 March 1948 by Belgium, the Netherlands, Luxembourg, France and the United Kingdom is considered the precursor to the NATO agreement. The treaty and the Soviet Berlin Blockade led to the creation of the Western European Union's Defence Organization in September 1948. However, participation of the United States was thought necessary in order to counter the military power of the USSR, and therefore talks for a new military alliance began almost immediately.

Such action as it deems necessary, including the use of armed force does not necessarily mean that other member states will respond with military action against the aggressor(s). Rather they are obliged to respond, but maintain the freedom to choose how they will respond. This differs from Article IV of the Treaty of Brussels (which founded the Western European Union) which clearly states that the response however often assumed that NATO members will aid the attacked member militarily. Further, the article limits the organization's scope to Europe and North America, which explains why the Falklands War did not result in NATO involvement.

The outbreak of the Korean War in 1950 was crucial for NATO as it raised the apparent threat level greatly (all Communist countries were suspected of working together) and forced the alliance to develop concrete military plans. The 1952 Lisbon conference, seeking to provide the forces necessary for NATO's Long-Term Defence Plan, called for an expansion to 96 divisions. However this requirement was dropped the following year to roughly 35 divisions with heavier use to be made of nuclear weapons. At this time, NATO could call on about 15 ready divisions in Central Europe, and another ten in Italy and Scandinavia. Also at Lisbon, the post of Secretary General of NATO as the organization's chief civilian was also created, and Baron Hastings Ismay eventually appointed to the post. Later, in September 1952, the first major NATO maritime exercises began; Operation Mainbrace brought together 200 ships and over 50,000 personnel to practice the defence of Denmark and Norway.

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