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)

On August 27, 1916, after Romania declares war on Austria-
Hungary, formally entering World War I, Romanian troops
cross the border of the Austro-Hungarian Empire into the
much-contested province of Transylvania.

By the outbreak of war in Europe in 1914, Romania had long
been at odds with Austria-Hungary over the issue of territory,
specifically Transylvania, which was ethnically Romanian but
then part of Hungary. Seeing Russia’s success against Austria
on the battlefields of the Eastern Front during the summer of
1916, Romania hoped to make an advantageous entry into the
war in order to realize long-held dreams of territorial expansion
and national unity.

Erich von Falkenhayn‘s cavalry entering Bucharest on
December 6, 1916.

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 Treaty of Annexation.

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. 

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