On May 13, 1846, the U.S. Congress overwhelmingly voted in favor
of President James K. Polk’s request to declare war on Mexico in a dispute over Texas.

Under the threat of war, the United States had refrained from
annexing Texas after the latter won independence from Mexico in
1836. But in 1844, President John Tyler restarted negotiations with
the Republic of Texas, culminating with a Treaty of Annexation.

The treaty was defeated by a wide margin in the Senate because it
would upset the slave state/free state balance between North and
South and risked war with Mexico, which had broken off relations
with the United States. But shortly before leaving office and with
the support of President-elect Polk, Tyler managed to get the joint resolution passed on March 1, 1845. Texas was admitted to the
Union on December 29. 

James Knox Polk
(November 2, 1795 – June 15, 1849)