Which country worked with the US after World War II to build rockets for the US space program?

Following World War II, several German rocket scientists emigrated to both the Soviet Union and the United States, assisting those countries in the Space Race of the 1960s.

Explanation:

I know this because we studied it in 2018.

the group i'm selecting: greeks.

a first group of greek immigrants came to the us in the 1850s. the second group came to america starting in the 1890s due to economic opportunities they didn't have back in their home caused by the hardships of ottoman rule. greek immigration was 90% male, many of them expected to work and return to greece after earning enough capital, but ultimately had to remain in the us after the greek genocide.