Answer:
The reason for this war was because of America's belief to move forward and expand also known as Manifest Destiny. You could say it was unjust but most American citizens agreed with the idea of Manifest Destiny.
(hope that helps a little)
Step-by-step explanation:
When the dust cleared, Mexico had lost about one-third of its territory, including nearly all of present-day California, Utah, Nevada, Arizona and New Mexico. The U.S was the victor and took a big chunk of Mexico's land.
To sum it up:
-Manifest Destiny caused this war
-America was victorious in taking Mexico's land