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.
Greece and Turkey joined the alliance the same year, forcing a series of controversial negotiations, in which the United States and Britain were the primary disputants, over how to bring the two countries into the military command structure. Meanwhile, while this overt military preparation was going on, covert stay-behind arrangements to continue resistance after a successful Soviet invasion ('Operation Gladio'), initially made by the Western European Union, were being transferred to NATO control. Ultimately unofficial bonds began to grow between NATO's armed forces, such as the NATO Tiger Association and competitions such as the Canadian Army Trophy for tank gunnery.
In 1954, the Soviet Union suggested that it should join NATO to preserve peace in Europe. The NATO countries, fearing that the Soviet Union's motive was to weaken the alliance, ultimately rejected this proposal. The incorporation of West Germany into the organization on May 9 1955 was described as "a decisive turning point in the history of our continent" by Halvard Lange, Foreign Minister of Norway at the time. A major reason for Germany's entry into the alliance was that without German manpower, it would have been impossible to field enough conventional forces to resist a Soviet invasion. Indeed, one of its immediate results was the creation of the Warsaw Pact, signed on May 14, 1955 by the Soviet Union, Hungary, Czechoslovakia, Poland, Bulgaria, Romania, Albania, and East Germany, as a formal response to this event, thereby delineating the two opposing sides of the Cold War.