The movement that enshrined American's right to peacefully assemble is known as the First Amendment.
The First Amendment of the United States Constitution states that "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press, or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."
Hence, the First Amendment ensures that the Americans have the rights or freedom to perform any form of religion, expression, assembly, and the right to petition without the Congress stopping them.
Therefore, in this case, the correct answer is First Amendment.
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