May 13, 1846

Mexican–American War

Polk signs and proclaims the U.S. war act against Mexico

Congress legalized a war that three battlefield clashes had already started.

13 May 1846: signed, then proclaimed

On 13 May 1846, President James K. Polk approved "An Act providing for the Prosecution of the existing War between the United States and the Republic of Mexico," making the congressional measure law.

He issued a proclamation the same day announcing the state of war and directing civil and military officeholders to discharge their wartime duties zealously. His secretary of state, James Buchanan, countersigned it.

Troops, money, and a finding of blame

The statute did more than declare. It authorized as many as 50,000 volunteers — serving twelve months from arrival at rendezvous, or until the war ended, unless sooner discharged — and appropriated $10 million.

Its wording held that a state of war existed because of an act of Mexico, and authorized Polk to employ the militia, naval and military forces. That assignment of blame was disputed, because the first clash had occurred in territory both countries claimed.

Three dates, and a war already running

"The United States declared war on 13 May" compresses three separate acts: the House voted 174–14 on 11 May, the Senate 40–2 on 12 May, and Polk signed on the 13th. John C. Calhoun abstained after failing to limit or slow the measure.

The shooting had begun well before any of that. The Thornton Affair was on 25 April, and Palo Alto and Resaca de la Palma were fought on 8 and 9 May, before the act became law.

Sources

Researched 23 Aug 2026 8 sources 2 audit passes

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

What the sources leave uncertain

May 13 is firmly documented as the date of presidential approval and proclamation.
“The United States declared war on May 13” is conventional shorthand. The House voted May 11, the Senate voted May 12 and Polk signed the act May 13.
The statute’s exact wording asserted that war already existed because of Mexico’s act. It therefore both recognized an existing state of war and authorized its prosecution.
The statutory assignment of blame to Mexico was disputed because the initial clash occurred in territory claimed by both countries.