The Mexican War was the most aggressive act of territorial expansion ever undertaken by the United States. By seizing the northern third of Mexico, the United States solidified its position as a transcontinental nation and significantly increased its total size. The conquest paid almost immediate dividends in another way as well, when gold was discovered in California the same year the United States took formal possession there. But the Mexican War was not without serious political costs. Many citizens had arisen to oppose what they regarded as an immoral war (Henry David Thoreau's famous essay "Civil Disobedience" was originally written as a protest against the Mexican War), and the acquisition of new territories reopened the explosive question of whether to allow the spread of slavery into those new territories.

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