Because it was America’s Manifest GOD given destiny to spread across this area of North América.
And what revelation imparted this knowledge of Divine Providence to you?
Because by your particular logic, one can easily justify the invasion of *any country*, by *any nation*, at *any time*.
(It also strikes me as odd, given your prior posting history regarding the Confederacy: Mexico had outlawed slavery in Texas in 1829, and the American settlers coming into Texas wanted to keep their chattel slaves, to point of making it part of their 1836 constitution when declaring themselves an independent republic.)