Areason that the united states and its allies formed nato was to:

a. invade the soviet union.
b. establish a single government in europe.
c. defend member nations against communist attacks.
d. promote free trade among its members.

C. defend member nations against Communist attacks.

Explanation:

North Atlantic Treaty Organization (NATO), military union set up by the North Atlantic Treaty (additionally called the Washington Treaty) of April 4, 1949, which tried to make a stabilizer to Soviet armed forces positioned in focal and eastern Europe after World War II. Its unique individuals were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom, and the United States. Joining the first signatories were Greece and Turkey (1952); West Germany (1955; from 1990 as Germany); Spain (1982); the Czech Republic, Hungary, and Poland (1999); Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia (2004); Albania and Croatia (2009); and Montenegro (2017). France pulled back from the coordinated military direction of NATO in 1966 however remained an individual from the association; it continued its situation in NATO's military command in 2009.

Defend member nations against Communist attacks.