Besides the Texas border, why did Americans want to fight the Mexican war?

To expand their territory

Explanation:

The Mexican-American War (1846-1848) marked the first U. S. armed conflict chiefly fought on foreign soil. It pitted a politically divided and militarily unprepared Mexico against the expansionist-minded administration of U. S. President James K. Polk, who believed the United States had a "manifest destiny" to spread across the continent to the Pacific Ocean. A border skirmish along the Rio Grande started off the fighting and was followed by a series of U. S. victories. 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.