What did the Rio Grande have to do with starting the Mexican American War in 1846?

Mexico claimed that the southern border of Texas was the Nueces River, but Texas claimed the true boundary was the Rio Grande, 150 miles further south.

On hearing of Mexico’s refusal to negotiate, President Polk stationed U.S. troops as deep as possible inside the disputed area, right on the banks of the Rio Grande.

If it was war he wanted, Polk did not have to wait long. General Taylor’s army arrived at the Rio Grande on March 28, 1846. On April 25, a Mexican force attacked, killing or wounding sixteen American soldiers. On May 13, at Polk’s request, the U.S. Congress declared war on Mexico.

A subordinate of Taylor’s named Colonel Hitchcock wrote: “It looks as if the government sent a small force on purpose to bring on a war, so as to have a pretext for taking California and as much of this country as it chooses.”