The four main causes of WWI were nationalism, imperialism, militarism, and the system of alliances. The match that lit the fire was the assassination of Franz Ferdinand, by Gavril Princip. Gavril Princip was a member of the terrorist organization the "Black Hand". After the assassination Austria-Hungary sent Serbia an ultimatum, but Serbia didn't agree to all 5 points that were on it. They didn't want to lose their pride, so they wouldn't let Gavril Princip be put on trial in Austria-Hungary. Austria-Hungary went to war with Serbia, so Slavic relatives of the Serbian people in Russia told the czar to help fight with Serbia. .
After Russia declared war on Austria-Hungary because they had attacked Serbia, Germany, as an ally of Austria-Hungary, declared war on Russia. Germany knew France would help Russia during the war, so they also declared war on France. Germany decided to attack France through Belgium in the time that Russia finished mobilizing. The Belgians had flat land, and Germany didn't want to go through their own borders with France because there were mountains there. Belgium didn't want to have their flowers destroyed, and because they got their own country in 1839 nobody could go through Belgium or attack them during a war. England promised Belgium that they would declare war on Germany as soon as they set foot on their land. Germany thought they had just signed "a scrap of paper", so they went through Belgium after all. During WWI there were 2 main sides. The Central Powers consisted of Germany, Austria-Hungary, Turkey and Bulgaria. The main Allies were France, Russia, the US, Italy, and Great Britain. Later in the war several countries joined the Allies. At first the USA was neutral, but after several incidents, like the sinking of the Lusitania and the Zimmermann telegram, they joined the side of the Allies. Other neutral countries were Belgium, Switzerland and Scandinavia.