The United States formally entered World War I a hundred years ago, on April 6, 1917.
The nation set aside its posture of neutrality nearly three years after the start of the bloody conflict that came to be known as the "Great War."
The war began in July 2014, a tumultuous period in Europe. Following the assassination in Sarajevo of Archduke Franz Ferdinand of Austria and his wife, Sophie, by a suspected Serbian nationalist gunman, the Central Powers — Germany, Austria-Hungary, the Ottoman Empire and Bulgaria — were pounding Russia, France and Britain.
The war was the most violent conflict in centuries, largely due to advances in the technology of warfare resulting from the Industrial Revolution. Armies were modernized, using tanks, high explosives, machine guns, trench warfare, airplanes and, to the horror of combat troops and civilians alike, poison gas — chemical weapons.
Shortly after Germany declared war on Russia and France in August 1914, President Woodrow Wilson told the U.S. Senate: "The United States must be neutral in fact, as well as in name. ... We must be impartial in thought as well as in action."
In November 1916, Wilson won re-election after campaigning on the slogan, "He kept us out of war!"
The United States remained neutral until a series of provocative acts by Germany, including deadly submarine assaults against U.S. merchant ships and a German U-boat's torpedo attack that sank the British cruise ship Lusitania off the coast of Ireland, shifted American public opinion about the war.
In early 1917, the United States learned about the Zimmermann Telegram, a coded message from German Foreign Secretary Arthur Zimmermann that tried to persuade Mexico to enter into an alliance against America, in return for Germany's help in restoring territory that Mexico lost to the United States.
That prompted Wilson to act.
In a speech to the U.S. Congress on April 2, 1917, Wilson made a compelling case to turn the United States into a world leader:
"We shall fight for the things which we have always carried nearest our hearts — for democracy, for the right of those who submit to authority to have a voice in their own governments, for the rights and liberties of small nations, for a universal dominion of right by such a concert of free peoples as shall bring peace and safety to all nations and make the world itself at last free."
Four days later, the United States declared war on Germany, formally entering World War I.
The first American troops arrived in France in June 1917. Less than a year later, by the spring of 1918, they were a significant fighting force for the Allies.
“They brought in the men, the material, the resources that these allies needed desperately,” historian Libby H. O’Connell said.
The arrival of fresh U.S. troops was a key factor in breaking a stalemate that had developed among the beleaguered European troops and helped end the war on November 11, 1918, known as Armistice Day. (The United States still celebrates November 11 as a national holiday, but in 1954 its name was changed to Veterans Day to honor all veterans of wartime service.)
The legacy of the Great War is hard to overstate.
Adjectives like cataclysmic, catastrophic and transformative are used by historians. In just four years, the world order was recast dramatically, killing off and creating new power brokers and ministates.
After only nine months in the war, the United States was suddenly a respected world power — and would go on to lead globally throughout the rest of the 20th Century. The country had become the world's largest economy shortly before the war broke out, and the challenge of mobilization spurred even faster growth.
The modernization of military effort that marked the Great War forever changed how wars are fought, and greatly magnified the cost of conflict. Combat alone resulted in a staggering loss of life - anywhere from 9 million to 14 million soldiers were said to have died during the war.
The end of World War I did not mark an end to Europe's troubles.
World War II broke out about two decades later - on September 1, 1939, when Nazi Germany invaded Poland. Many historians blame the second war on the weaknesses inherent in the Versailles Treaty of 1919 that ended World War I.