What was a main issue that started the Mexican War?

Well, basically it was the land, Mexicans claimed the Nueces River, and the U. S. claimed the Rio Grande, and when the troops met at the Rio Grande, the Mexican Army opened fire, starting the war.

When the Republic of Texas became a US state, the new border was disagreed upon. The US claimed Texas south to the Rio Grande River, while Mexico claimed the border that was further north at the Nueces River. There was a conflict between the Mexican Army and the US Army, with US soldiers killed or taken prisoner. And so, this event started the Mexican War.