The north Atlantic Treaty organization combines the military strength of the United States, Europe (mainly western European countries) and Canada. These countries were the founding members and signed the treaty in Washington D.C. on August 24th, 1949. The treaty was signed as a military cooperation pact of western countries against the Soviet countries, who signed the Warsaw Pact in 1955 as a response. Over the years the NATO has included former soviet countries as Lithuania, Latvia, and Estonia.