A reason that the United States and its allies formed NATO was to invade the Soviet Union.
establish a single government in Europe.
defend member nations against Communist attacks.
promote free trade among its members.

Answer : Defend member nations against communist attacks.

Nato was formed after WWII, and the aftermath of the soviet union. The united states and other nations were mostly concerned about the soviet union and their communist attacks, so they decided to form NATO to prevent this and promote peace among nations.

C )  defend member nations against Communist attacks.

Explanation:

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It was called The progressive era because it was a time of widespread social activism and political reform across the United States. It lasted from the 1890s through the 1920s.