FDR Signing Declaration of War Against Japan.

June 18, 1812: The day after the Senate followed the House of Representatives in voting to declare war against Great Britain,
President James Madison signed the declaration into law—and
the War of 1812 began.

The American war declaration, opposed by a sizable minority
in Congress, had been called in response to the British
economic blockade of France, the induction of American
seaman into the British Royal Navy against their will,  and
the British support of Indian tribes along the Great Lakes  
frontier.     

A faction of Congress known as the “War Hawks” had
been advocating war with Britain for several years and
had not hidden their hopes that a U.S. invasion of Canada
might result in significant territorial land gains for the
United States.         

James Knox Polk (November 2, 1795 – June 15, 1849)

On May 13, 1846, the U.S. Congress overwhelmingly votes in
favor of President James K. Polk’s request to declare war on
Mexico in a dispute over Texas.

Under the threat of war, the United States had refrained from
annexing Texas after the latter won independence from Mexico
in 1836.

But in 1844, President John Tyler restarted negotiations with
the Republic of Texas, culminating with a Treaty of Annexation.

But shortly before leaving office and with the support of President
-elect Polk, Tyler managed to get the joint resolution passed on
March 1, 1845. Texas was admitted to the Union on December 29. 

llustration: General Zachary Taylor rides his white horse at
the Battle of Palo Alto near present-day Brownsville, Texas,
8 May 1846.

On September 3, 1939, in response to Hitler’s invasion of Poland,
Britain and France, both allies of the overrun nation declare war
on Germany.

The first casualty of that declaration was not German—but the
British ocean liner Athenia, which was sunk by a German U-30
submarine that had assumed the liner was armed and belligerent.

There were more than 1,100 passengers on board, 112 of whom
lost their lives. Of those, 28 were Americans, but President
Roosevelt was unfazed by the tragedy, declaring that no one
was to “thoughtlessly or falsely talk of America sending its
armies to European fields.” The United States would remain
neutral.

Franklin Delano Roosevelt
(January 30, 1882 – April 12, 1945)

British ocean liner SS Athenia was sunk when a torpedo
from the German submarine U-30 sank her in the Western Approaches.

James Knox Polk (November 2, 1795 – June 15, 1849)  Polk 
was the 11th president of the United States, serving from
1845 to 1849.

On May 13, 1846, the U.S. Congress overwhelmingly voted in favor
of President James K. Polk’s request to declare war on Mexico in
a dispute over Texas.

Under the threat of war, the United States had refrained from
annexing Texas after the latter won independence from Mexico
in 1836. But in 1844, President John Tyler restarted negotiations
with the Republic of Texas, culminating with a Annexation Treaty.

The treaty was defeated by a wide margin in the Senate because it
would upset the slave state/free state balance between North and
South and risked war with Mexico, which had broken off relations
with the United States. But shortly before leaving office and with
the support of President-elect Polk, Tyler managed to get the joint resolution passed on March 1, 1845. Texas was admitted to the
Union on December 29.

(February 22–23, 1847)