In the 1800's the very large country of Russia was a very backward country, with a large population. It also had a large army, but it was badly equipped and poorly led. Russia was quite a poor country which hardly had any industry at all. Russia, along with France and Britain, was worried about Germany's growing strength.
In 1904 France and Britain became allies. Fear of Germany's apparently aggressive policies led these countries to join in military alliance. Because Russia was also worried about Germany's growth, France and Russia made a similar defensive agreement and became allies in 1907. This alliance between Britain, France and Russia was known as the Triple Entente.
Because Russia was a very backward country the other two countries in the Triple Entente planned to help Russia become more powerful. Some ways that they planned to do this were by funding its army to help it improve, by funding it's transport network to improve communications and to help the army get from one place to another more quickly and also by helping it to become more industrial.
Germany was surrounded by the three countries in the Triple Entente. Germany's leaders were worried by this and they started to make military plans, in case France and Russia jointly declared war on them. The countries in the Triple Entente knew that they were in a good position. They desperately wanted to weaken Germany and stop it becoming a very powerful country. Another country that surrounded Germany, but was not against it was Austria-Hungary.
Previously Germany had formed an alliance with Austria-Hungary. This was because the Austrians felt threatened by the Russians and by nationalism in the Balkans (an area of land in south-east Europe). The Russians wanted the Austrian empire (made up of many different nationalities) to fall apart because the Russians wanted a chance to expand their influence into Europe. Russia had a long-term aim of gaining ports on the coast of the Mediterranean Sea. The Austrians were particularly worried by Serbia. Austria owned the land to the west of Serbia, called Bosnia. However many of the people living in Bosnia were Serbs. Austria did not want Serbia to expand but Serbia wanted a coastline. Russia was friends with Serbia and thought that their friendship would increase their influence in the Balkans. This put the Russians in direct conflict with the Austrian government. Serbia was made up of mainly Slavs and wanted to bring them all together into one country. Austria-Hungary didn't agree with the idea of Slavs being united because it was concerned that the large number of Slavs that lived in its lands would not want to live in its lands anymore and would join with Serbia.
So, Austria-Hungary had conflicts with the Russians about the Balkans and also with Serbia because of Serbia's friendship with Russia and Serbia's ideas about Panslavism (uniting the Slavs).Previously Austria-Hungary had an alliance with Germany. The Austrians wanted this alliance because they felt threatened by the Russians and by nationalism in the Balkans.
In 1906 Germany's war minister Count Alfred von Schlieffen who was worried about this made military plans to defeat both the French and the Russians, should they declare war on Germany. This plan was called the Schlieffen Plan. He thought that if there was going to be a war then the huge country of Russia would be slow to mobilize because they would have such long way to go with a very backward transport system. Because Germany thought that it would take Russia a couple of months to have it's forces ready, he planned to defeat the French first and then go and deal with the Russians. It was planned that the Germans would invade France through the neutral country of Belgium, so that the French wouldn't realize what was happening until the Germans got close to their country. The Germans then planned to surround Paris, win the battle and move the bulk of the army away again to fight the Russians. This plan was worked out precisely down to every last detail. Even the train times were planned down to every last minute.
The Austrian-Hungarian government strongly opposed the Serbs ideas about Panslavism. Some Serbs began to carry out acts of terrorism in Austria-Hungary because they disagreed with the government. Several secret societies were formed in Serbia and were dedicated to throwing the Austrians out of the Balkans. One of the well-known terrorist group was called the Black Hand.
On June 28th 1914 the heir to the Austrian-Hungarian throne and his wife visited Sarajevo, the capital of Bosnia. The Black Hand saw this as an opportunity to protest about the Austrians taking over Bosnia to stop Serbian expansion. Several members of the Black Hand were waiting for the archduke when he visited Sarajevo. Firstly a bomb was thrown at his car, which miraculously did not hit him. After this incident Franz Ferdinand was going to leave Sarajevo. His driver turned the car around and as he was doing so, one of the members of the Black Hand, Gavrillo Princip fired two shots at the car. The first killed Archduke Franz Ferdinand and the second killed his wife, Countess Sophia.
After this event Austria-Hungary was very annoyed with Serbia and blamed them for the killing of the archduke. They accused the Serbian government of helping Princip. This then annoyed Serbia because it was not actually their country, but a small group of Bosnian Serbs. Austria-Hungary declared war on Serbia. Before doing so they checked that their German allies would stand by them. The Austrian-Hungarians attacked Belgrade, the capital of Serbia.
On the 29th July the Russian army got ready to help Serbia defend itself against the Austrians. The next day Germany, being in the Triple alliance with Austria-Hungary, sent a threatening message to the Russians ordering them not to help Serbia.
Because Austria-Hungary didn't like Serbia this caused Germany to declare war on Russia. Germany also began to move its army towards France and Belgium, because France was a member of the Triple Entente along with Russia. The French army was aware of this and was put on war alert, ready to fight a German invasion.
On the 3rd and 4th of August Germany invaded France through neutral Belgium. Britain, being on France's side, ordered Germany to leave Belgium. When Germany refused, Britain declared war.
On the 6th August Austria-Hungary declared war on Russia, because Russia was friendly with Serbia, whom Austria-Hungary had already declared war on and Russia had tried to defend. Also Austria-Hungary had had previous conflicts with Russia about the Balkans.
The German's meticulously planned Schlieffen Plan actually failed. It was a lot weaker in practice than the German's had expected because they were slowed down by fierce resistance in Belgium. The British succeeded at the Battle of Mons and pushed the Germans to the east of Paris. The French had sent the Russians money to develop the army, who mobilized more quickly than the Germans had expected. The Germans had to send two divisions of the army to try to stop them on the Eastern front.
There was a deadlock on the Western front by the end of September 1914 because both sides dug trenches which eventually stretched from the channel coast to the Swiss border. The war of movement was over.
I blame the long term causes more than the short term causes for starting the war. This is because a long time before the war there were more meaningful disputes over land and power. The short term causes such as the assassination of the archduke just sparked off the war.