World War I

World War I (WWI or WW1), also called the First World War, began on July 28, 1914 and lasted until November 11, 1918. It was a global war and lasted exactly 4 years, 3 months and 2 weeks. Most of the fighting was in continental Europe. 
Soldiers from many countries took part, and it changed the colonial empires of the European powers. Before World War II began in 1939, World War I was called the Great War, or the World War. Other names are the Imperialist War and the Four years War. There were 135 countries that took part in World War I, and nearly 10 million people died while fighting.
Before the war, European countries had formed alliances to protect themselves. However, that made them divide themselves into two groups. When Archduke Franz Ferdinand of Austria was assassinated on June 28, 1914, Austria-Hungary blamed Serbia and declared war on it. Russia then declared war on Austria-Hungary, which set off a chain of events in which members from both groups of countries declared war on each other. 
The two sides were the Allied Powers (mainly Russia, France, the British Empire and later the United States) and the Central Powers (mainly Germany, Austria-Hungary and the Ottoman Empire).
There was fighting in many different fronts (areas). The French and the British fought the Germans on the Western Front in France and Belgium. Germany had tried to defeat France quickly with the Schlieffen Plan but were stopped in the First Battle of the Marne. Most of the fighting on the western front was trench warfare. The Russians fought the Germans and Austro-Hungarians on the Eastern Front in Central and Eastern Europe. Fighting on the eastern front was not trench warfare as in the west, but mobile warfare. The other main areas of fighting were in the Middle East, in the Gallipoli region of the Ottoman Empire and between Italy and Austria-Hungary. Fighting also took place in Africa, China, at sea and in the air. World War I was the first major war in which tanks, airplanes and submarines (or U-boats) were important weapons.
In 1917, the Russian Revolution led to Russia leaving the war in March 1918. Also in 1917, the United States entered the war, but it took a year for most of its army to arrive. From the time of the Russian departure, before the American arrival, the Germans launched a huge attack in March 1918 to try to win the war, but it failed. 
From August to November 1918, the Allied Powers won a large victory against the Germans during the Hundred Days Offensive. Austria-Hungary and the Ottoman Empire then agreed to stop fighting. The German government collapsed, and a new government was forced to agree to stop the fighting on November 11, 1918. Basically, the Germans ran out of men. 
Also, the influenza pandemic of 1918 had a big influence. It killed many people in Europe: it lasted for three years, from January 1918 to December 1920. About 500 million people were infected across the world which then had a population of 1.80 billion people.
The war was ended by the signing of many different treaties, the most important being the Treaty of Versailles. It also led to the creation of the League of Nations, which was meant to prevent wars. People were shocked by the size of the war, how many people it killed and how much damage it caused. They hoped it would be "the war to end all wars". Instead it led to another, larger, world war 21 years later.
Background.
By 1914, trouble was on the rise in Europe, and many countries feared invasion from others. For example, Germany was becoming increasingly powerful, and the British Empire saw that as a threat. Countries formed alliances to protect themselves, but that divided them into two groups. Germany and Austria-Hungary had been allies since 1879 and formed the Triple Alliance with Italy in 1882. France and Russia became allies in 1894 and later formed the Triple Entente with Britain.
In 1908, Austria-Hungary had taken over Bosnia, a region next to Serbia. Some of those who lived in Bosnia were Serbs and wanted to be part of Serbia. The Black Hand was an organization that agreed with the idea and sent men to kill Archduke Franz Ferdinand of Austria while he was visiting Sarajevo, the capital of Bosnia. All of them failed to kill him with grenades while he passed through a large crowd. However, one of them, a Serbian student, Gavrilo Princip, shot him and his pregnant wife with a pistol.
Austria-Hungary blamed Serbia for the assassination. Germany supported Austria-Hungary and promised full support in case of war. Austria-Hungary sent the very harsh July Ultimatum to Serbia and threatened war unless Serbia agreed to all of its terms. Many historians think that Austria-Hungary already wanted a war with Serbia. Serbia agreed to most of the conditions, but Austria-Hungary still declared war on Serbia, which quickly led to a full-scale war. The countries had allies that entered the war in a matter of days.
Russia joined the war to help Serbia because both Serbs and Russians are Slavs. Russia began to assemble its army (mobilize), which caused Germany to began to mobilize its forces. Germany declared war on Russia in support of Austria-Hungary and began to carry out its plan to fight a war in Europe.
Germany is in the middle of Europe and so it had to worry about enemies to its east and west. Germany's plan was the quick defeat of France in the west before Russia was ready to fight, then Germany would move its armies to the east to face Russia. However, Germany could not quickly invade France directly because France had put so many forts on the border. That made Germany invade the neighboring country of Belgium to invade France through the undefended French-Belgian border.
Britain then joined the war and said that it was to protect Belgium. However, some historians think that even if Germany had stayed out of Belgium, the British would have still joined the war to help France.
Soon, most of Europe became involved. The Ottoman Empire (now Turkey) joined the war on the side of Germany and Austria-Hungary. It is not clear why the Ottomans entered the war, but they had become friendly to Germany. Italy had allied with Germany and Austria-Hungary but refused to help them since those countries had not been attacked first. Italy noted that Austria-Hungary had attacked Serbia first. Also, Italy had long disliked Austria-Hungary and wanted to take part of its land. It later joined the war for the Allies because they had promised land across the Adriatic Sea.
Germany against Russia.
Germany was afraid that because Austria-Hungary had attacked Serbia, Russia would attack Austria-Hungary to help Serbia. That made Germany feel that it had to help Austria-Hungary by attacking Russia first.
The problem was that Russia was allied to France, and the Germans thought that the French might attack them to help Russia. That made Germany decide that it could win the war by quickly attacking France first. The Germans could mobilize first since they had a list of all men who had to join the army, where they had to go, and the times of every train that would carry them to where they had to go.
France was slower in doing the same thing. The Germans thought that if they attacked France first, they could knock it out of the war before Russia could attack them.
Russia had a large army, but Germany mistakenly thought that it would take six weeks to mobilize and a long time before they could attack the Central Powers. However, the Russian Army mobilized in ten days and drove deep into Austria-Hungary.
In the east, the Russians attacked the Germans. The Russians pushed back the Germans, but the Germans then defeated the Russians at the Battle of Tannenberg.
United Kingdom against Germany.
The United Kingdom quickly joined the war and said that it was to protect Belgian neutrality. Germany passed through Belgium to reach Paris before Russia could mobilize and open up a second front. On August 4, 1914, the British declared war against Germany in support of Belgium. The British Empire was larger than any other empire and ruled over a quarter of the world. If Germany won in France, it might take British and French colonies and become the most powerful and largest empire in the world.
Britain was also worried about Germany's growing military power. Germany was developing its large army into one of the most powerful in the world. The British Army was quite small, but the Royal Navy was the largest and the best in the world, which in the 19th century was enough to keep other naval powers from attacking Britain.
Germany was a land power, and Britain was a sea power. However, Germany was building a large navy, which was seen as a threat to Britain. Germany's generals had decided that the best way to defeat France was to go through Belgium using a plan called the "Schlieffen Plan" by German Army Chief of Staff Alfred Von Schlieffen. The Germans could then attack the French army at the north side and the south side at the same time. The German Army went into Belgium on August 4. The same day, the British declared war on Germany. The British had said in 1839 that they would not let Belgium be invaded, and they kept their promise.
When the Germans got to the Belgian city of Liège, the Belgians fought very hard to stop them from coming into the city. The Germans finally pushed the Belgians out of the city, but it had taken longer than the German generals had planned. The Germans then attacked the north side of the French army. The French and the British moved men up to fight the Germans because the Belgians had fought so long at Liège. However, the Germans pushed the French back at the frontiers, and the British held the Germans back at Mons, but they also fell back to join up with the retreating French Army until they were stopped at the river Marne. This was the First Battle of the Marne or "Miracle of the Marne".
The British government might have decided differently if anyone had foreseen how long the war would last and what a terrible price Britain would pay.
Most people thought that the war would be short. They thought the armies would move around quickly to attack each other and one would defeat the other without too many people getting killed. They thought the war would be about brave soldiers and did not understand how war had changed. Only a few people, such as Lord Kitchener said that the war would last long.
Ottoman Empire.
The Ottomans joined the war because they were secretly allied to Germany, and two Ottoman warships, manned by German Navy personnel, bombarded Russian towns.
Britain fought against the Ottomans only for supporting Germany and had no other problem with them. However, by fighting them in the Mesopotamia region in what is now Iraq, in the Arabian Peninsula and in other places, Britain defeated them with the help from the British Indian Army. After the war ended, Britain got some Ottoman areas and added them to the British Empire.
Greece went into the war because its leader supported the Allied cause. Greece and Serbia had become independent, but many Greeks still lived in lands that had once been Greek but were now Ottoman. Having recently won the Balkan Wars, the Greeks wanted especially to control other land to the north that was under Bulgarian and Ottoman rule and so they declared war. The Ottomans killed most of the Greek Army as they tried to regain parts of the Ottoman Empire. Another war started when the Greeks bombed a train. The Ottomans swept the Greeks back into their own territory. From then on, the Greeks never again declared war, but the Ottomans had one of the largest armies in the world.
Bulgaria against Serbia and Greece.
Bulgaria, like Greece and Serbia, had been part of the Ottoman Empire but later became independent. Bulgaria claimed a lot of Ottoman land. The Serbians and the Greeks felt cheated because they felt that the land belonged to them and took back the land. That angered Bulgaria and led to it becoming an Ottoman ally. Bulgaria declared war on Serbia and Greece but lost the war.
Trench warfare.
Trench warfare killed many soldiers. New weapons, such as machine guns, and long-range artillery had an increased rate of fire that cut down huge numbers of soldiers during mass charges, a tactic left over from older warfare. The men on both sides took spades and dug holes because they did not want to be killed. The holes joined up into trenches until the lines of trenches went all the way from Switzerland to the North Sea. In front of the trenches was barbed wire to cut anyone who tried to climb over it and land mines to blow up anyone who tried to cross. Late in the war, poison gas also was an important weapon.
The new machine guns, artillery, trenches, and mines made it very difficult to attack. The generals had fought many wars without them and so they ordered their armies to attack in the old style of marching in rows, which allowed the enemy to shoot them down easily. At the 1916 Battle of the Somme, 60,000 British men died in a single day, one of the bloodiest days in the history of the British army. Late in the war, the British and French invented tanks and used them to attack entrenched Germans, but they could not make enough of them to make a big difference. The German Empire responded by making their own tanks but they had even fewer. The Germans also invented special Sturmabteilung tactics to infiltrate enemy positions. This was also too little, too late.
The British used whistles to communicate to other soldiers and so before they shelled the German trenches, they would sound the whistle. However, the Germans caught on to this tactic and so after the shelling, when the British soldiers came to finish off the German soldiers, the Germans were ready with their machine guns because they knew that the British were coming.
Airplanes.
Airplanes were first used extensively in World War I. They were first used for reconnaissance, and pictures of enemy land were taken to help direct artillery. Generals, who are military leaders, were using airplanes as an important part of their attack plans at the end of the war. World War I showed that airplanes could be important war weapons.
Airplanes in World War I were made of wood and canvas, a type of rough cloth. They did not last long and could not fly very fast at the beginning of the war. They could only fly up to 116 kilometers per hour, or 72 miles per hour. At the end of the war, they could fly up to , much slower than modern planes. Guns were put on planes for the first time during the war. Pilots, people who fly the plane, used the guns to shoot enemy planes. One pilot used metal sheets to armor his airplane. Other pilots began using metal sheets too. Pilots also made their airplanes better with machine guns, guns that shoot bullets much faster. That made fighting harder and more dangerous between airplanes.
Pilots had to wear certain clothes when flying an airplane in World War I because they flew high, where the air is cold. The pilots' clothes kept them warm and protected them from the wind and cold. Pilots wore a leather coat to protect their bodies, a padded helmet and goggles, which are large glasses with special lenses, to protect their head and face. They wore a scarf around their neck to keep the wind from blowing against their neck when they turned their head.
United States against Germany.
The German leaders decided to use submarines, called U-boats, from the German word "Unterseeboot" (meaning "underwater boat"). The U-boats attacked passenger ships such as RMS Lusitania carrying civilians to the United Kingdom. They did not follow the laws of war because the British could easily destroy them otherwise. America was selling weapons to Germany's enemies but not to Germany and so was not neutral since the US had taken sides during the conflict. Many American and British noncombatants were killed by the submarines although they were not fighting.
Germany also wrote a secret telegram note to Mexico in code that suggest that both countries work together to attack the United States. This note is called the Zimmerman Telegram because it was sent by Arthur Zimmerman. It offered Mexico land in the Southwestern United States that the United States had taken in the Mexican-American War. British spies found out about the note and told the United States. The Americans became angry, and many decided that their country should enter the war against Germany. The Zimmermann Telegram and the sinking of American ships by German U-boats made the United States on April 6, 1917 declare war against Germany and join the Allies.
Russian Revolutions.
The defeat of Russia on the Eastern Front caused unrest there, which led to the Russian Revolutions.
They made Russia fight both Germany and the Bolsheviks at the same time. That meant Russia wanted to end the war against Germany. 
That made Russia want to pay many German marks to make stop the fighting between it and Germany so that it could focus on fighting the Bolsheviks.
First Russian Revolution.
In 1917, there was a revolution in Russia. Tsar Nicholas II had to give up power and a new government was established. At first, it was thought that Russia would fight harder now that he was gone. However, the Russians did not want to fight anymore because there was too little food, weapons or adequate roads to supply its army. The war had been putting burdens on the Russians, and many of them were poor and hungry. They began to hate their new government because it would not stop the war.
Second Russian Revolution.
Another revolution happened in Russia in which the Mensheviks lost to the Bolsheviks (these were two rival groups of communists). The leader of the Bolsheviks was Vladimir Lenin (1870-1924), a communist who followed the ideas of Karl Marx. The new Bolshevik government asked the Germans for peace and signed the Treaty of Brest-Litovsk with the Central Powers in March 1918 at the city of Brest-Litovsk. Lenin later died in 1924 and was replaced by Joseph Stalin.
The Germans and Russians stopped fighting. Russia gave Germany land in Eastern Europe and the Baltic Sea, including the Baltics, Poland, Ukraine and Belarus. Also, Finland gained its independence by the treaty.
Treaty of Versailles.
After the war, the Germans had to agree to the Treaty of Versailles, which stated that Germany had to pay approximately $31.5 billion in reparations and had to take full responsibility for the war.
Also, the countries of the world were to come together to make an international organization to stop future wars: the League of Nations. The United States Senate did not agree with the League even though it was the idea of United States president Woodrow Wilson. Although Wilson tried to tell the American people to agree to join the League, the United States never joined it.
Problems with the treaty in Germany, where it was hated, later led to World War II in 1939.