5 Things you need to know about World War I: the key players, causes, and effects--which we're still living with 100 years later.

• Author: Liulevicius, Vejas Gabriel
• Position: TIME PAST: 1914

It was dubbed the "Great War," but for most Americans today, World War I might better be called "the forgotten war." It didn't have the tragedy of brother fighting brother during the Civil War, the glory of the D-Day invasion of World War II, or the internal strife of Vietnam to burn it into the nation's memory. And yet, the First World War, in which 10 million people died, was actually a crucial war for the United States.

World War I marked America's entry onto the world stage and the beginning of its status as a superpower. How the war ended laid the foundations for the rise of Communism and Fascism, paving the way for World War II (1939-45) and the Cold War that followed. Its effects in the Middle East still make headlines today, from the bloody civil war in Syria to ethnic clashes in Iraq and the ongoing territorial conflict between Israelis and Palestinians.

The last surviving veteran of WWI, an American from West Virginia, passed away in 2011. But on the 100th anniversary of the war's outbreak, the world is taking a look back at its legacy and why it mattered so much.

1 How the assassination of one man led to a worldwide war

On June 28, 1914, Archduke Franz Ferdinand's chauffeur took a wrong turn in Sarajevo (see map, p. 21) and a 19-year-old Serb terrorist named Gavrilo Princip shot and killed the heir to the throne of the Austro-Hungarian Empire and his wife. Princip believed the empire controlled territories that should belong to Serbia, but his two shots soon triggered a world war lasting four years. Other assassinations hadn't led to war, so why did this one?

Because of Europe's tangled system of alliances, the conflict was almost waiting to happen. Various countries were bound by treaties to protect one another, and the assassination was like a spark that set in motion a chain reaction. When the dust had settled, the so-called "Central Powers," led by Germany, Austria-Hungary, and the Ottoman Empire, were at war with the "Allied Powers," including Russia, Great Britain, and France.

At first, Europeans welcomed the war. Young men rushed to volunteer, worried that the fighting would end before they'd have their chance for glory and adventure. During what became known as "August Madness" in 1914, cheering crowds in London, Berlin, Paris, Vienna, St. Petersburg, and other cities celebrated the outbreak of the war. The British poet Rupert Brooke wrote, "Now God be thanked Who has matched us with His hour" (he later died in the war). Among the crowds in Munich, Germany, was a young Adolf Hitler, at loose ends as a failed artist. He later wrote that he experienced the outbreak of the war as a "salvation from the vexing feelings of youth.... Overcome by turbulent enthusiasm I fell to my knees and thanked heaven with an overflowing heart that I had the fortune to live in this time."

2 Why the U.S. entered the War in 1917

President Woodrow Wilson at first promised to keep the U.S. out of this European quagmire and announced that the nation was "too proud to fight." Many Americans were glad to be far from the slaughter. During his 1916 campaign for re-election, Wilson's slogan was, "He kept us out of...