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H.R. 145, A bill providing for the prosecution of the war between the United States and Mexico, May 1846

Congress declared war against Mexico in 1846, claiming that a state of war existed “by the act of the Republic of Mexico.” That “act”—Mexican forces wounding U.S. soldiers in a skirmish—occurred after President James K. Polk sent troops into a disputed border area. This printed House bill shows revisions by the Senate before passage.

Records of the U.S. Senate, National Archives and Records Administration

H.R. 145, A bill providing for the prosecution of the war between the United States and Mexico, May 1846

The Annexation of Texas

For much of the first half of the 19th century, there was a balance in Congress between Northern free states and Southern slave states. In 1836 Texas, which permitted slaveholding, declared independence from Mexico and sought annexation to the United States. Mexico threatened war over U.S. annexation of Texas, and in Congress the issue of annexation inflamed debate regarding slavery’s expansion. After the Senate rejected an annexation treaty, Congress annexed Texas in 1845 by a joint resolution and declared war with Mexico the following year.