[147] He also sent assurance that the United States would defend Texas, and would fix its southern border at the Rio Grande, as claimed by Texas, rather than at the Nueces River, as claimed by Mexico.[2][148][149] Public sentiment in Texas favored annexation. In July 1845, a Texas convention ratified annexation, and thereafter voters approved it.[150] In December 1845, Texas became the 28th state.[151] However Mexico had broken diplomatic relations with the United States on passage of the joint resolution in March 1845; now annexation escalated tensions as Mexico had never recognized Texan independence.[152]

Mexican-American War[edit]
Main article: Mexican–American War
Road to war[edit]
Following annexation in 1845, Polk began preparations for a potential war, sending an army to Texas, led by Brigadier General Zachary Taylor.[153] American land and naval forces were both ordered to respond to any Mexican aggression but to avoid provoking a war. Polk thought Mexico would give in under duress.[154]

Polk's presidential proclamation of war against Mexico
Polk hoped that a show of force would lead to negotiations.