In the 1840s the United state and Mexico fought overin

The Mexican-American War, which took place in 1846 - 1848 was a result of the US annexation of Texas.

At the time, Mexico still owned Texas, it was a part of its territory until America incorporated that state into its own territory in 1945. Obviously, this sparked an armed conflict between two countries which ended with Mexico accepting the annexation after the US paid a lot of money for it.