After Texas became the 28th state on December 29, 1845, President James K. Polk turned his attention to California, hoping to acquire the territory from Mexico before any European nation did so.
His main interest was San Francisco Bay as an access point for trade with Asia. Earlier in 1845, President Polk had sent diplomat John Slidell to Mexico to purchase California and New Mexico for $24–30 million. Mexico had refused to receive Slidell, citing a technical problem with his credentials.
Thus, in January of 1846, President Polk sent troops under General Zachary Taylor into the area between the Nueces River and the Rio Grande River as a way to increase pressure on Mexico to negotiate.
Mexico continued to rebuff John Slidell, and he returned to Washington in May of 1846.
President Polk regarded this treatment of his diplomat as an insult and an “ample cause of war”, and prepared to ask Congress for a declaration of war.
Meanwhile, Zachary Taylor crossed the Rio Grande and briefly occupied Matamoros, Tamaulipas.
Taylor continued to blockade ships from entering the port of Matamoros as well. As a result, mere days before President Polk intended to make his request to Congress, he received word that Mexican forces had crossed the Rio Grande area and killed eleven American soldiers.
Declaration of war
On May 11, 1846, President Polk made this his casus belli (cause for war), and in a message to Congress stated that Mexico had “invaded our territory and shed American blood upon American soil.”
Congress approved Polk’s declaration of war against Mexico on May 13, 1846.
Although Mexican President Paredes’s issuance of a manifesto on May 23, 1846 is sometimes considered the declaration of war, Mexico officially declared war against the United States on July 7, 1846.
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