American soldiers fire artillery in France during World War I, a war the United States was reluctant to enter. (Library of Congress)

They thought it would be over within months. They were wrong.

Instead, the world was at war for four long years. Powerful nations took sides — the Allies versus the Central Powers. From 1914 to 1918, they battled across Europe and into Asia and Africa. The United States supplied the Allies with goods but tried to stay out of the fighting. But in April 1917, 100 years ago this week, it joined the Allies (led by Great Britain and France) by declaring war on Germany and Austria-Hungary.

World War I brought an end to one way of fighting and the start of another. Soldiers on horseback, called cavalry, were replaced by tanks. Planes and submarines also saw action.

By the war’s end, more than 16 million people had died. The shocking numbers of dead and wounded led some to call it “the war to end all wars.”

They were wrong about that, too.

Men sign up to serve in World War I at a U.S. Navy recruiting station. (National Photo Company/Library of Congress)
America enters the fight

U.S. kids don’t learn much about World War I in school. Why? For starters, the war was a long time ago, and it wasn’t fought here. Also, America joined late, so its losses were limited.

America got into the war mostly because German U-boats (subs) kept sinking unarmed ships in the Atlantic. Germany knew this risked pulling the United States into the war but thought it could defeat the Allies before U.S. troops or warships were ready.

Instead, America’s entry changed the course of the war. In addition to troops, the United States provided arms, tanks, ships, fuel and food to its friends. This aid helped the Allies win.

As in any war, unexpected heroes arise. Here are two American ones:

Stubby showed up at an Army training camp in 1917 and ended up accompanying the troops to France. He earned several awards for saving lives and capturing a German soldier in World War I. (Larry Gates/National Museum of American History)
One dog’s life

You could say Stubby joined the Army in 1917. The terrier pup with a short tail showed up one day at a training camp in Connecticut and became the troops’ mascot. He learned their bugle calls and drills.

When the men left to join the fighting, they smuggled Stubby onto their ship. He was discovered after they arrived in France but was allowed to stay after lifting his right paw to his forehead and saluting the commanding officer!

On the battlefield, Stubby’s keen nose picked up the tiniest trace of poison gas, and he barked until everyone donned gas masks. (Dogs and horses also had masks.) Stubby’s keen ears helped him find wounded soldiers in the field. He listened for English being spoken, then barked until medics came to help. He even captured an enemy soldier by biting him on the leg.

Stubby took part in 17 battles and was made a sergeant. He has been the subject of several books and a movie that is due out in 2018.

Sergeant Alvin C. York was an unlikely war hero. He wanted to stay home and care for his many siblings but he ended up capturing 132 Germans near Cornay, France, where he’s pictured here after World War I. (National Archives)
‘Don’t want to fight’

That’s what Alvin York wrote to explain his feelings about going to war. York’s father had died when Alvin was young, and he was needed at home in Tennessee to help raise his eight younger siblings.

But the Army had other ideas and shipped him to France. In October 1918, York led an attack on a German machine-gun nest. With just a rifle and pistol, he killed several German soldiers and captured 132. When he got back to camp, he wrote later, a general said: “ ‘Well, York, I hear you have captured the whole … German army.’ And I told him I only had 132.”

Though awarded the Medal of Honor — the highest U.S. military award — York shunned celebrity and went home to Tennessee. Back on the farm, the war hero with the third-grade education raised money to build a good high school for his neighbors’ kids.