波爾克最後在試圖執行議會決議 - 納入德州同時取得加州與新墨西哥州土地的指令時，引發與墨西哥的戰爭（該地已於1836年宣佈脫離墨西哥獨立）。

墨西哥拒絕接納波爾克派駐的使節（外交官）約翰史萊德（John Slidell），當時波爾克以四千萬美金的價格要購得全數土地、包含解決德州西方與墨西哥邊界的爭議問題，墨西哥總統也希望達成共識，但就在將要完成交易的時候、部分墨西哥的軍隊將領群起反抗他的決定。

波爾克指派札卡裡泰勒（Zachary Taylor）將軍和四千名美國大兵到大雷恩河（Rio Grande River）備戰，就在墨西哥部隊過河與美方交戰時，議會同意了波爾克的建議、在1846年5月13日宣戰。

在1848年2月2日，墨西哥終於同意簽署和平條約、並接受美國以一千五百萬美金交換雙方爭奪的土地，美國取得的土地擴大到該國百分之二十五的領土。

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?

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