Polk ended up starting a war with Mexico when he tried to implement the congressionally approved annexation of Texas (which had declared its independence from Mexico in 1836) and acquire California and New Mexico.
Mexico refused to receive Polk's envoy (diplomat), John Slidell, who offered up to $40 million to settle the whole deal through territorial purchase, including resolving the dispute over Texas's western border with Mexico. Mexico's president had been willing to settle, but when it came time to finalize the deal a number of Mexican army officers revolted against him.
Polk sent General Zachary Taylor and 4,000 U.S. soldiers to the Rio Grande River and after a small group of Mexican troops crossed the river to engage the Americans, Congress followed Polk's wishes by declaring war on May 13, 1846.
On February 2, 1848, after many battles, Mexico finally agreed to a peace treaty and $15 million from the United States in exchange for ownership of the contested land. This acquisition increased the size of the nation by 25 percent.
How did Polk avoid war with Britain in order to acquire the Oregon Territory?