Until World War II, it was known as the Great War.
Archduke Franz Ferdinand was heir to the Austria-Hungarian throne when he was assassinated on June 28, 1914, in Sarajevo by a Serbian nationalist. That led Austria to threaten Serbia with an ultimatum that Serbia rejected.
Behind this were great power machinations. Germany played the biggest part, giving Austria assurance that when it acted, Germany would have its back. However, Russia supported Serbia, which activated the larger alliance system, including the Russian-French alliance.
When the United States entered on April 6, 1917, the war was at a stalemate; the trenches in France had hardly moved from fall 1914 to the beginning of 1917. Several events led to US involvement. When Woodrow Wilson ran for reelection in fall 1916, he campaigned on having kept the country out of the war. The American people thought they had chosen a platform of peace. But in February 1917, Germany declared unrestricted submarine warfare (to try to stop the flow of supplies to Britain), which was a big threat to the US. Another factor was the February 1917 revolution in Russia that turned that country from an autocracy into a republic. That was very important for Wilson: now the war was a battle of democracies against monarchies.
The US didn't begin sending substantial numbers of troops until a year after the declaration of war. Mainly because of its disastrous offensive in summer 1917, the Russian effort completely fell apart. Soldiers began deserting, the Bolsheviks seized power, and Russia withdrew from the war in March 1918. That allowed Germany to conquer Ukraine and send troops from the Russian front to France for a big push to win the war. It was only then that US forces became important. Even more critical than their presence on the battlefield was the realization of what the future would hold. US forces could be multiplied many times, so if Germany didn't win quickly, there was no way that it could win. The German offensives made gains, but at a heavy cost, so the German leadership decided to put their hopes in Wilson for a just peace, signing the Armistice on November 11, 1918. On June 28, 1919—exactly five years after the assassination of Archduke Ferdinand—the Treaty of Versailles was signed, formally ending the war.
World War I, which claimed 17 million lives, marked the transition of a world of empires to one of nation-states. It planted the seeds for the Great Depression and a revanchist Germany, and gave rise to communism. On the eve of World War I, it was widely assumed that a global-scale war could not happen. We live with that assumption today. Reflecting upon how it all went unthinkably wrong a century ago just might help us to avoid the unthinkable in our lifetimes.