Why did the Mexican War begin?

The Mexican War between the United States and Mexico began with a Mexican attack on American troops along the southern border of Texas on Apr. 25, 1846. Fighting ended when U.S. Gen. Winfield Scott occupied Mexico City on Sept. 14, 1847.

Leading causes of the Mexican War:

1. The U.S. annexation of Texas (Dec., 1845)
Mexico had warned it would regard annexation as an act of war. When it took place, Mexico did not declare war, but broke diplomatic relations.

2. The Boundary Dispute.
The United States maintained that the southern border of Texas was formed by the Rio Grande, but Mexico argued that the traditional boundary was at the Nueces River farther north.

3. The California Question.
President Polk of the U.S. clearly wanted to expand the country to the Pacific Ocean by taking control of California and lands in New Mexico, part of which was claimed by Texas.

4. Monetary Claims against Mexico.
The United States had extracted a promise from the Mexican government to pay $3 million to cover the claims of American citizens who had lost property during turmoil and revolution. Mexico defaulted on those payments and the American creditors pressed their government for action.

美國與墨西哥之間的戰爭，起因於墨西哥對美國1845年併吞德克薩斯的憤怒，以及關於德克薩斯究竟以努埃塞斯(Nueces)河為界(墨西哥要求)還是以格蘭德河為界(美國要求)的爭執。

1845年美國併吞德克薩斯後不久，墨西哥於1845年3月與美國斷絕關係。9月，波克總統派斯利德爾(John Slidell)祕密出使墨西哥城，擬就有爭議的德克薩斯邊界進行談判，解決美國對墨西哥的領土要求，並準備以30,000,000美元的代價購買新墨西哥和加利福尼亞。墨西哥官員拒絕接待。波克總統獲悉這一怠慢舉動後，下令泰勒將軍指揮部隊進占有爭議的地區。
1846 5月13日，美國國會批准宣戰，戰爭遂爆發。

Mexican War, 1846–48, armed conflict between the United States and Mexico.
While the immediate cause of the war was the U.S. annexation of Texas (Dec., 1845), other factors had disturbed peaceful relations between the two republics. In the United States there was agitation for the settlement of long-standing claims arising from injuries and property losses sustained by U.S. citizens in the various Mexican revolutions.

Another major factor was the American ambition, publicly stated by President Polk, of acquiring California, upon which it was believed France and Great Britain were casting covetous eyes. Despite the rupture of diplomatic relations between Mexico and the United States that followed congressional consent to the admission of Texas into the Union, President Polk sent John Slidell to Mexico to negotiate a settlement. Slidell was authorized to purchase California and New Mexico, part of which was claimed by Texas, and to offer the U.S. government's assumption of liability for the claims of U.S. citizens in return for boundary adjustments.

When Mexico declined to negotiate, the United States prepared to take by force what it could not achieve by diplomacy. The war was heartily supported by the outright imperialists and by those who wished slave-holding territory extended. The settlement of the Oregon boundary dispute (June, 1846), which took place shortly after the official outbreak of hostilities, seemed to indicate British acquiescence, for it granted the United States a free hand.