World War I -- aka the Great War -- broke out in 1914 and significantly shaped the rest of the 20th Century, abroad and here at home. April 6, 2017, marks 100 years since the U.S. got involved in the international conflict.

Centennial events will be unfolding across the country this year to remind Americans of the sacrifices made during that war and how it shaped many of the policies and problems we still have today. So, if your World War I history knowledge is a little rusty, here are some key facts about the Great War and America's involvement in it.

WHAT STARTED WORLD WAR I?

The war officially began toward the end of July 1914 after Franz Ferdinand, the archduke of Austria, was assassinated by a Serbian nationalist in Bosnia, which was part of the Austria-Hungarian Empire. The general feeling was that Serbia, a country with which Bosnia had religious tensions, was responsible for the assassination, so the empire declared war on the Serbs.

Here's how it played out from there:

-- France and England tried to mediate the situation, to no avail.

-- Germany -- an ally of Austria -- refused to intervene, even though Russia had pleaded for it to do so.

-- Russia then mobilized part of its army to help Serbia, its ally.

-- Germany and Russia declared war on each other.

-- France mobilized its troops, declaring it would not remain neutral in a German-Russian battle.

-- Germany invaded France's neutral neighbor, Belgium, in an attempt to quickly invade France and capture Paris by surprise.

-- Great Britain declared war on Germany to stand beside its allies, Belgium and France.

-- Italy and the U.S. later joined the war on the side of Britain, France and Russia, also known as the Allies.

Which leads us to ...

WHY DID THE U.S. ENTER THE WAR SO LATE?

America had chosen to remain neutral for much of the war. But after a German submarine (known as a U-boat) sank the British cruise liner Lusitania in May 1915, killing 123 Americans as they innocently crossed the Atlantic, America grew concerned over Germany's unrestricted submarine warfare tactics. Still, the U.S. stayed neutral for two more years.

On April 6, 1917 -- 100 years ago -- the U.S. finally joined the fray. President Woodrow Wilson cited two reasons for the declaration of war: Germany had tried to get Mexico to join in an alliance against the U.S. (which failed), and Germany had also violated a pledge to suspend its unrestricted submarine warfare in the North Atlantic and the Mediterranean.

Our role in helping the Allies win the war and mediating its aftermath led to the rise of the U.S. as a global power.

WE WERE TECHNICALLY NOT PART OF THE ALLIES

While the U.S. joined the war on the side of the Allies, we technically entered the war as an associated power. That meant we weren't bound to honor pre-existing agreements among the Allied powers, most of which focused on post-war redistribution of territories.

VETERANS DAYS IS A PRODUCT OF THE GREAT WAR

The armistice that ended the war's hostilities took effect on Nov. 11, 1918. A year later, President Woodrow Wilson declared it Armistice Day, a time to honor those who took part in the war. A few years later, Nov. 11 became a federal holiday. In 1954, after the events of World War II and the Korean War, President Dwight D. Eisenhower signed legislation changing its name to Veterans Day.

GERMANY'S POST-WAR PUNISHMENT LED TO THE RISE OF HITLER

While Armistice Day ended hostilities, the Treaty of Versailles signed on June 28, 1919 -- exactly five years after the archduke's assassination -- established the terms of post-war peace and implemented strict punishment on Germany at the wishes of France and the U.K.

Germany accepted responsibility for the war and, therefore, had to pay about $37 billion in reparations to the Allies. It also had to give up about 10 percent of its pre-war territory and all of its overseas possessions. Its military was limited in size, and two resource-rich areas of the country were put under control of the League of Nations (and later exploited).

Germans grew to resent the harsh conditions imposed on them, which caused political instability and an economic collapse. That led to the rise of fascism in Europe and the Nazi Party in Germany, led by Adolf Hitler, who broke the Treaty of Versailles in 1933.

WAR OF 1812'S FAMED FORT MCHENRY WAS BUSIER DURING WWI

Fort McHenry in Baltimore, Maryland, is famous for what occurred there during the War of 1812. The Chesapeake Bay, just off the fort's shores, was where Francis Scott Key penned the "Star-Spangled Banner" during a naval battle.

But the fort was actually much more active during World War I. An Army hospital had been built around it and, over the course of the war, more than 20,000 sick and wounded soldiers passed through it.

THE LAST REMAINING U.S. WWI VETERAN DIED IN 2011

Army Cpl. Frank Buckles, a U.S. soldier from Charles Town, West Virginia, died on Feb. 27, 2011, at 110 years of age. He had enlisted in the Army when he was 16 and was the last surviving World War I "doughboy," which was a term used to describe American Expeditionary Forces troops who had been deployed to Europe during the conflict.

There are plenty more interesting facts about World War I to share. But we've got an entire year to remember and celebrate, so keep checking back -- we'll be sharing more stories throughout the centennial.