Russia joined the war on Serbia's side because the people of Serbia were Slavic just like Russia and the Slavic countries had agreed to help each other if they were attacked. Since the Russian Empire was a large country it had to move soldiers closer to the war, but Germany feared that Russia's soldiers would also attack Germany. Russia did not like Germany because of things Germany had done in the past to become stronger. Germany declared war on Russia, and began to carry out a plan created long before to fight a war in Europe. Because Germany is in the middle of Europe, Germany could not attack to the east towards Russia without weakening itself in the west, towards France. Germany's plan involved quickly defeating France in the west before Russia was ready to fight, and then moving her armies to the east to face Russia. Germany could not quickly invade France directly, because France had put a lot of forts on the border, so Germany invaded the neighboring country of Belgium to then invade France through the undefended French/Belgian border. Great Britain then joined the war, saying they wanted to protect Belgium. 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 they entered or chose to fight on their side, but they had become friendly to Germany. Although Italy was allied with German and Austria-Hungary, they had only agreed to fight if those countries were attacked first. Italy said that because Austria-Hungary had attacked Serbia first, they did not need to fight. They also had started to dislike Austria-Hungary, so in 1915, Italy joined the war on the Allied Powers' side.
Germany was allied with Austria-Hungary. Russia was allied with Serbia. The German government was afraid that because Austria-Hungary had attacked Serbia, Russia would attack Austria-Hungary to help Serbia. Because of this, Germany felt it had to help Austria-Hungary by attacking Russia first, before it could attack Austria-Hungary.
The problem was that Russia was also friends with France, and the Germans thought the French might attack them to help Russia. So the Germans decided that they could win the war if they attacked France first, and quickly. They could mobilize very quickly. They had a list of all the men who had to join the army, and where those men had to go, and the times of every train that would carry those men to where they would have to fight. France was doing the same thing, but could not do it as quickly. The Germans thought that if they attacked France first, they could 'knock France' out of the war before Russia could attack them.
Russia had a big army, but Germany thought that it would take six weeks to mobilize and a long time before they could attack the Central Powers. That wasn't true, because the Russian Army mobilized in ten days. Also, the Russians drove deep into Austria.
Britain was allied with Belgium, and became quickly involved in the war. Britain had promised to protect Belgian neutrality. Germany passed through Belgium to reach Paris before Russia could mobilize and open up a second front against them. On August 4, 1914, Britain declared war against Germany in support of Belgium. Britain had the biggest empire (it ruled over a quarter of the world). If Germany conquered France, it might take Britain and France's colonies and become the most powerful and biggest 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. The British Royal Navy was the largest and best in the world, and in the 19th century that was enough to keep other naval powers from attacking. Germany was a land power, and Britain was a sea power. But now the Germans were building a large navy. This was seen as a threat to Britain. However, the decision to declare war was taken under its alliance with Belgium in the Treaty of London (1839). The Government might have decided differently. No-one foresaw how long the war would last, and what the terrible costs would be.
The Ottoman Empire (Turkey) went into the war because it was secretly allied to Germany and two Turkish warships manned by German Navy personnel bombarded Russian towns.
Britain also fought against Turkey because the Ottoman Empire was supporting Germany. Britain did not have any animosity towards the Turks. However, by fighting the Turks in the Mesopotamia region (in what is now called Iraq), in the Arabian Peninsula and other places, Britain was able to defeat them with help from the British Indian Army. Later, after the War ended, Britain was able to get some areas from the old Turkish empire which was breaking up, and to add 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 were once Greek but were now in the Turkish Ottoman Empire. Having recently won the Balkan Wars, the Greeks especially wanted to control other land to the north that was under Bulgarian and Turkish rule, so they declared war. Turkey killed most of the Greek army as the Greeks tried to regain parts of Turkey. Another war started when the Greeks bombed a train. Turkey swept Greece back into their own territory. From then on the Greeks never again declared war, while Turkey had one of the biggest armies in the world.
Bulgaria, like Greece and Serbia, was owned by Turkey before Bulgaria broke away from Turkey. Bulgaria claimed a lot of Turkish land as belonging to Bulgaria. The Serbians and Greeks felt cheated because they felt the land belonged to Greece or Serbia. The Greeks and Serbians took back the land which angered Bulgaria and led to the country becoming allies with Turkey. They declared war on Serbia and Greece, but Bulgaria lost this war.
The Russian Revolution makes Russia fight Germany and the Bolsheviks at the same time. Russia surrendered to Germany due to the fact that the Russians were fighting against the Soviets as well. It needed to get out of the war, so they payed Germany lots of German marks to make them stop fighting between them so they could focus on fighting the Soviets.
Most people thought 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 — they did not understand how war had changed. Only a few people, for example Lord Kitchener said that the war would take a long time.
In the beginning of the war, Italy was in the Central Powers. But then Italy changed the side of the Entente Powers because they had promised land across the Adriatic sea.
Germany's generals had decided that the best way to defeat France was to go through Belgium using a plan called the "Schlieffen Plan". This was invented by the German Army Chief of Staff, Alfred Von Schlieffen. They 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 the 4th. On the same day, Great Britain started a war on Germany, because Britain was a friend of Belgium. The British had said some time before, in 1839, that they would not let anyone control Belgium, and they kept their promise.