What was the cause and effect of the u. s. mexican war?

Causes American Imperialism, and Mexico's unstable gov't deterred them from being able to focus on the problem of the imposing Uncle Sam on their backyard. Effects The Mexican-American War had many long-term effects. The first and most obvious is the vast territory Mexico was forced to give up to the United States, including the present-day states of California, Nevada, Utah, Arizona, and New Mexico. California also became a state after the war, following the short-lived 'California Republic' which had been declared in Sonoma by American settlers in revolt against Mexico (their flag is now the California state flag). The leading American general of the war, Zachary Taylor, would later be elected the 12th President. However, the the most important event (at least in American history) that resulted was the American Civil War.