Belligerents:

The United States of America.
Mexico.

Cause:

Texas seceded from Mexico in 1836. The independence of Texas was recognized by the United States, but the proposal that the new state should be admitted into the Union was declined. A strong support for the annexation of Texas in the interests of slavery grew up, more especially in the Southern states, and in December 1844 resolutions were passed in both Houses, and it was formally enrolled as a new state.

Occasion:

The Mexican Government still claimed Texas as a province, and its annexation by the United States was considered an act of hostility. The Americans had suffered long under continued acts of insult and spoliation on the part of the Mexicans, and were therefore prepared to fight.

Course of the War:

The Americans under Taylor invaded Mexico, won battles at Palo Alto and Resaca and captured Monterey. In 1847 there were more victories, the Mexicans under Santa Anna being everywhere defeated. The Americans entered Mexico City on September 14th. After further fighting peace was proclaimed at Washington in July 1848.

Political Result:

By the Treaty of Guadalupe-Hidalgo, February 2, 1848, Mexico ceded the whole of Texas, New Mexico, and Upper California. The United States surrendered their other conquests.

Remarks:

So far as Texas was concerned, the political opinion in the United States was divided, and that division was to become more serious as time went on. On the other hand, Mexico was a troublesome neighbour, and has continued ever since to be the cause of disturbance and dispute.