that Europe already had a defensive military alliance prior to NATO?
In 1948 Belgium, France, Luxembourg, the Netherlands and the United Kingdom signed the Treaty of Brussels creating the Western Union for the defence of Western Europe. Under the Brussels Treaty a common defence structure known as the Western Union Defence Organisation or WUDO was established with its headquarters at Fontainebleau, France. Field Marshal the Viscount Montgomery of Alamein was the WUDO's senior officer as Chairman of the Commanders-in-Chief Committee, but he was very critical of this loose committee-type structure and called for the creation of a supreme commander.
The WUDO continued to exist until NATO created an effective military command structure in 1951 and took over responsibility for the defence of Western Europe, at which time the headquarters, personnel and plans of the WUDO transferred to Allied Command Europe. The Treaty of Brussels was revised considerably in 1954 as part of the negotiations that led to the admission of the Federal Republic of Germany into NATO, and the Western Union was transformed into the Western European Union (WEU).