World War I
At the end of the 1800s, many countries around the world were angry that France, Britain, and the United States were controlling most of the world and taking whatever they wanted to make themselves richer. Other people wanted to be richer too. In Central Asia, Russia tried to take over the Ottoman Empire and Afghanistan, but the British and French fought the Crimean War to stop them. Austria wanted to take over Eastern Europe. Naples, Tuscany, Lombardy, Venice and Rome united into the new country of Italy in 1870. Prussia, Bavaria, and Saxony united in 1871 to form Germany, and then Germany began to try to get colonies and control sea trade like Britain.
France, Britain, and the United States began to build stronger and stronger armies to defend their power. The new factories and chemistry inventions of the late 1800s let these countries make new kinds of weapons: lots of very accurate guns, machine guns, bombs, poison gasses, ships, and airplanes. Soon Russia, Germany, and Italy also built big strong armies. But once they had built these strong armies with all kinds of new cool weapons, generals were eager to use them.
Dead Romanian soldiers (1916)
World War I started in 1914 when Austria invaded Eastern Europe, and right after that Germany invaded France. Spain, which didn't have the new weapons and had allies on both sides, stayed out of the war. Millions of men died: most of the young men of England, France, and Germany died. Many died in horrible ways, poisoned by gas.
Nobody got what they wanted out of World War I. Instead of getting stronger, Germany became weaker, and the winners forced both the Austrian Empire and the Ottoman Empire to break up into a lot of smaller, weaker countries. In Russia, soldiers angry about losing the war joined a revolution and demanded the redistribution of land.
But the winners also came out of the war weaker than before. Britain soon lost control of Canada, Australia, Egypt, and New Zealand, and Ireland and India moved closer to independence too. France began to lose control of Indochina, North Africa, and Madagascar. Following the pattern Napoleon had used in Haiti, and the Spanish had used in Morocco, the Allies demanded that Germany pay a huge fine to them, which left the Germans angry and poor.
Not only that, but as the war ended, moving millions of soldiers under bad conditions caused a huge flu epidemic that killed many more people than the war itself did, all over the world.