Final answer:
In 1949, the United States and 11 other nations formed NATO through the North Atlantic Treaty to guard against Soviet aggression, establishing a system of collective defense that played a key role during the Cold War. The correct option is (A).
Step-by-step explanation:
In 1949, the United States and 11 other nations signed an agreement known as the North Atlantic Treaty as a countermeasure against the potential threat of aggression by the Soviet Union. This agreement led to the formation of the North Atlantic Treaty Organization (NATO), a collective defense alliance in which member states agreed that an armed attack on any one of them in Europe or North America would be considered an attack against them all.
The Western Bloc, consisting of the capitalist democratic countries in NATO, opposed communist expansion in Europe by the Eastern Bloc, composed of the Soviet Union and its allies. To counter this, the Soviet Union later formed its own alliance, the Warsaw Pact, with Eastern European countries. These alliances reflected a divided Europe and became a central feature of the Cold War's geopolitical landscape.