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)

William McKinley (January 29, 1843 – September 14, 1901)

On this day in history, April 20, 1898, President William McKinley
asked Congress to declare war on Spain.

The declaration requested by the 25th U.S. president was in
response to an ongoing conflict between Spain and Cuba, the
latter located less 100 miles off the coast of Florida, as that
island country struggled for independence from Spain.

The U.S. Office of the Historian said on April 20, Congress
passed a joint resolution that acknowledged Cuban
independence.

Congress also authorized McKinley to use whatever military
measures he deemed necessary to guarantee Cuba’s
independence, the U.S. Office of the Historian noted.

 
President Woodrow Wilson addresses congress to declare
war on Germany.

The United States plunged ferociously into World War I following
a vote in the House of Representatives on this day in history,
April 6, 1917.

President Woodrow Wilson requested a declaration of war against Germany before a joint session of Congress on April 2, as the
United States faced both the threat of unrestricted submarine
warfare from Germany and the potential of its alliance with
Mexico to seize vast swaths of the American West.

The Senate voted in favor of the war declaration two days later. 

On April 2, 1917, President Woodrow Wilson asked Congress
to send U.S. troops into battle against Germany in World War I.

Four days later, Congress obliged and declared war on
Germany.

Thomas Woodrow Wilson
(December 28, 1856 – February 3, 1924)