You may have heard that World War I started when a Serbian terrorist shot an Austro-Hungarian archduke in Sarajevo, Bosnia, in 1914. It's true but much more complicated than that.
How did World War I start and end?
For one thing, the Serbians were angry with the Austro-Hungarian Empire (yes, it was a combination Austria and Hungary) for annexing Bosnia (even though Bosnia still technically belonged to the Ottoman Empire). The Austro-Hungarians worried about the Serbs' potentially uniting all the Slavs in southeastern Europe, which could threaten the Hungarian part of their empire. Russia was mad at Austro-Hungary, too. The Russians saw that part of Europe, the Balkans, as their sphere of influence.
Russia mobilized troops, which caused the Germans (allies of the Austro-Hungarian Empire) to declare war on both Russia and its ally France. In 1914, the Germans cut through neutral Belgium on the way to attack France. As relations between Britain and Germany were strained by an undeclared race between them for naval superiority, German troops crossing into Belgium gave the British an excuse to enter the war.
Eventually, Japan, Serbia, Portugal, Romania, Italy, and China joined the Allies (Britain, France, and Russia). The Ottoman Empire cast its lot with the Central Powers (Germany and Austro-Hungary).
In 1915, the British passenger liner Lusitania was sunk by a German submarine. In all, 1,195 passengers, including 128 Americans, lost their lives. Americans were outraged and put pressure on the U.S. government to enter the war. President Woodrow Wilson campaigned for a peaceful end to the war, but in 1917, when the Germans announced that their submarines would sink any ship that approached Britain (even more passenger ships), Wilson declared that America would enter the war and restore peace to Europe.
By 1918, German citizens were striking and demonstrating against the war. The British navy blocked German ports, which meant that thousands of Germans were starving and the economy was collapsing. Then the German navy suffered a major mutiny. After German Emperor Kaiser Wilhelm II abdicated on November 9th, 1918, the leaders of both sides met at Compiegne, France. The peace armistice was signed on November 11th.
By the time it was said and done, four empires — the Russian, the Ottoman, the German, and the Austro-Hungarian had collapsed because of the war.
In 1919, The Treaty of Versailles officially ended the War. But the Treaty was brutal towards Germany — requiring that Germany accept full responsibility for causing the war; make reparations to some Allied countries; surrender some of its territory to surrounding countries; surrender its African colonies; and limit the size of its military.
The Treaty also established the League of Nations to prevent future wars. The League of Nations helped Europe rebuild and fifty-three nations joined by 1923. But the U.S. Senate refused to let the United States join the League of Nations, and as a result, President Wilson (who had established the League) suffered a nervous collapse and spent the rest of his term as an invalid.
Although Germany joined the League in 1926, continuing resentment because of The Versailles Treaty caused them to withdraw (along with ally Japan) in 1933. Italy withdrew three years later. The organization subsequently proved helpless to stop German, Italian, and Japanese expansionism.
Some might argue that World War I never had an effective ending, but the battles just stopped. World War II never would have happened if not for World War I, because had the Germans not been beaten down so badly by the demands of The Treaty of Versailles, Adolf Hitler may not have risen to power in the 1930s and convinced the Germans to fight regain their dignity and place in the world.