What was the source of conflict between the United States and Mexico before 1846?

Annexation of Texas and more.

Explanation:

It stemmed from the annexation of the Republic of Texas by the U.S. in 1845 and from a dispute over whether Texas ended at the Nueces River (the Mexican claim) or the Rio Grande (the U.S. claim).

note: you can also look this up, it is likely much faster than using .

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