The 90 mm gun was the US Army's primary heavy anti-aircraft gun from just prior to the opening of World War II into 1946, complemented by small numbers of the much larger 120 mm M1 gun. Both were widely deployed in the United States postwar as the Cold War presented a perceived threat from Soviet bombers. The anti-aircraft guns were phased out in the middle 1950s as their role was taken over by surface-to-air missiles such as the MIM-3 Nike Ajax.