Texan Annexation. 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.
The Boundary Dispute. Regardless of its status (was it an American state or a rebellious Mexican province?), 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.
The California Question. President Polk clearly wanted to expand the country to the Pacific Ocean by taking control of California and lands in the Southwest a prime example of the prevailing Manifest Destiny mentality.
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.
---- Selected Quotes ----
Quotes regarding Causes of the Mexican War.
By Henry Clay This is no war of defense, but one of unnecessary and of offensive aggression. It is Mexico that is defending her firesides, her castles and altars, not we.