Many immigrants to the U. S. During the last quarter of the twentieth century had escaped by boat from which communist country?

Vietnam

Explanation:

Vietnam was originally divided into two (North and South Vietnam) in an attempt by the Western powers to weaken communism in the country. This eventually led to the Vietnam War of which the U.S. was heavily involved.

Towards the end of the war and as the U.S. was pulling out, the Communists gained the upper hand and as they began to capture more territory, people began fleeing South Vietnam by boat in order to escape communism. A lot of them eventually resettled in the U.S. in the last quarter of the 20th century.

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