The United States entered World War I on April 6, 1917.

The War was decided in the first twenty days of fighting, and all that happened afterwards consisted of battles which, however formidable and devastating, were but desperate and vain appeals against the decision of Fate. 

~ Sir Winston S. Churchill (wiki) (1874-1965) (Preface to Spears, Liaison 1914)

Napoleon had said it was rare to find generals willing to fight battles. The curse [of World War I] was that so few could do anything else. 

~ T. E. Lawrence ("of Arabia," 1888-1933)  (The Science of Guerrilla Warfare)

When every autumn people said it could not last through the winter, and when every spring there was still no end in sight, only the hope that out of it all some good would accrue to mankind kept men and nations fighting. When at last it was over, the war had many diverse results and one dominant one transcending all others: disillusion. 

~ Barbara Tuchman (1912-1989) (The Guns of August, "Afterward") 

Today is the 100th anniversary - the centennial - of the entry of the United States into World War I on April 6, 1917, when the House of Representatives passed the declaration of war proposed four days earlier by President Woodrow Wilson (1856-1924) (wiki) and adopted by the Senate on the 4th. The United States only joined the Allies after nearly three years of war, provoked beyond endurance by Germany's unrestricted submarine warfare campaign and the German "Zimmermann telegram" (wiki) - intercepted by British intelligence - which promised Mexico the return of her "lost territories" in the southwest United States in return for an alliance with a victorious Germany. 

Ultimately nearly two million American troops joined French and British counterparts on the Western Front and provided the final impetus for breaking the stalemate that had lasted there since late 1914. The United States lost over 53,000 men killed or missing in action, plus 204,000 wounded.* In his call for a war declaration, President Wilson noted,

Zimmerman Telegram

"It is a fearful thing to lead this great peaceful people into war ... We shall fight for the things 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."

Alas, these noble aspirations were largely unrealized, and the primary legacy of World War I - the worst calamity to afflict Western Civilization in our times - was World War II.

* N.B. The U.S. dead were only a small percentage of the total number of those killed on both sides - 5-1/2 million. It's also notable that the United States military suffered even more deaths to other causes, mostly the influenza pandemic that struck in 1918. 

A brief documentary on the entry:

From the BBC: 

In this collection of archive footage and interviews, the introduction and training of United States soldiers to the Allied cause in the First World War is discussed.

The text above is adapted from Ed's Quotation of the Day, only available via email - leave your email address in the comments if you'd like to be added to his list. Ed is the author of Hunters and Killers: Volume 1: Anti-Submarine Warfare from 1776 to 1943 and Hunters and Killers: Volume 2: Anti-Submarine Warfare from 1943.

If you're interested in further information on the subject there are hundreds of books and films - the best books I know of (and unlike Ed, I'm no expert) are Barbara Tuchman's The Guns of August (which won a Pulitzer back when they meant something) and John Keegan's The First World War.