Question

Why was the Mexican-American War knows as "Mr. Polk's War"?

Answer 1

Because he provoked it

Explanation:

There was territory in dispute between the U.S., and Mexico, preceding the war, there were attempts by the U.S. to buy the territory, but were unable to, due to Mexican pride and internal instability following their independence.

Basically, Polk sent Zachary Taylor (a general), with a detachment of troops, into the claimed territory, which, naturally, angered the Mexicans.

The Mexican general Santa Anna (with 2,000 soldiers) attacked a 70 men U.S. patrol, in what was known as the "Thornton Affair", which killed 11 Americans, and gave Polk a casus belli* (cause for war), and then war was declared on May 13, 1846.

*The captain of that 70 men patrol was named Seth Thornton, thus the name.