Washington, D.C. (JFK+50) On May 13, 1846, the Congress of the United States approved President James K. Polk's request for a war declaration on Mexico. The official state of war came after ongoing troubles over the border between Mexico and Texas.
In April 1846, President Polk sent a military force led by General Zachary Taylor to the mouth of the Rio Grande River "to meet a threatened invasion of Texas by Mexican forces." Those forces crossed over and attacked American troops.
In his war message of May 11th, the President said...
"The cup of forbearance has been exhausted...but...Mexico has passed the border of the United States, has invaded our territory and shed American blood upon American soil."
On August 8, 1846, President Polk requested $2,000,000 from the Congress to fund the war with Mexico. The Mexican daily newspaper El Republicano responded with these words...
"A government...that started a war without a legitimate motive is responsible for all its evils and horrors. Such is the case of the United States government for having initiated the unjust war its waging...today."
After the United States annexed Texas in 1845, Mexico claimed the border was at the Nueces River while the United claimed the border was farther south at the Rio Grande. Thus, the area where Mexican troops attacked U.S. troops was actually within disputed territory. President Polk accordingly based his words "American blood shed on American soil" on the U.S. claim that the southern border of Texas was at the Rio Grande.
"A Mexican Viewpoint on the War with the United States," by Jesus Velasco-Marquez, U.S.-Mexican War 1846-1848, www.pbs.org/
"James K. Polk's Message on War with Mexico, May 11, 1846," New Perspectives on the West, www.pbs.org/