The French, retreating in 1812—though according to tactics they should have separated into detachments to defend themselves—congregated into a mass because the spirit of the army had so fallen that only the mass held the army together.
There are no more uses of "congregate" in the book.
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People congregated outside of City Hall to protest.
It was in the quiet at the end of the day, when we began to congregate in the open space before climbing into our caves.