Final answer:
The annexation of Texas and boundary disputes, particularly the claim by the U.S. that the Rio Grande was the border with Mexico, led to heightened tensions and was one cause of the U.S.-Mexican War.
Step-by-step explanation:
One cause of the U.S.-Mexican War was the dispute over the territorial boundary between the U.S. and Mexico, which escalated significantly after the U.S. annexed Texas. The annexation of Texas in 1845 stirred tensions, with the U.S. claiming the Rio Grande as the border and Mexico insisting on the historic boundary at the Nueces River. The Mexican-American War was, therefore, partly provoked by President Polk's order of U.S. troops into the disputed territories. Conflicts in these regions led President Polk to contend that Mexico "has invaded our territory and shed American blood upon American soil," which helped galvanize U.S. support for declaring war on Mexico in 1846.