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The Mexican–American War
Texas had broken from Mexico in 1836 and was annexed by the United States in 1845, which Mexico never recognised — and the southern boundary was disputed. The war was contested inside the United States while it was still being fought; a young congressman named Abraham Lincoln challenged its justification, and Thoreau went to jail for refusing to pay tax towards it. The decisive consequence was internal: every new territory had to be placed on one side or the other of the slavery question, and that balance could not hold. The war is therefore the direct prelude to 1861.
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