Question

Why were many Americans in favor of going to war with Mexico?

Answer 1

What time period are you talking about?

Answer 2

What I know of is because the mexicans gained control of West in the Americas and the Colonist wanted to expand America due to the Gold rush also. The result of the war was The Treaty of Guadalupe Hidalgo specified the major consequence of the war: the forced Mexican Cession of the territories of Alta California and New Mexico to the United States in exchange for $15 million. In addition, the United States assumed $3.25 million of debt owed by the Mexican government to U.S. citizens. Mexico accepted the loss of Texas and thereafter cited the Rio Grande as its national border.

So America gained control of California, New Mexico and Texas :-)

hope this helped