Question

why did the united states join the north atlantic treaty organization in 1949?
to improve relations with communist countries in eastern europe
to protect against the soviet union and the spread of communism
to share a common currency across all nations within the treaty
to create a policy of using military force to combat terrorism

Explanation:

The North Atlantic Treaty Organization (NATO) was formed during the Cold War. The U.S. joined to counter the Soviet Union's influence. Communism was seen as a threat. NATO was a military alliance. Sharing a common currency wasn't its goal (the euro came much later). Improving relations with Eastern - bloc communist countries was not the aim. Combating terrorism as a main policy wasn't relevant in 1949.

Answer:

to protect against the Soviet Union and the spread of communism