Germany (a country that started/lost II World Wars
I don't want to be nagging or something, but the First World War was not started by Germany, but by Austria-Hungary and actually all the European participants except Belgium and Luxembourg were to blame for it's coming to be.
Austria-Hungary: simply wanted to show their muscles in revenge against small Serbia where it did not have any muscles.
Bulgaria: simply wanted revenge for the lost Second Balkans War and felt betrayed by it's fellow Slavic countries
France: had been stirring up things in Europe ever since the lost Napoleontic wars and European politics (Metternich and the Concert of Europe) were aimed at preventing France from ever developing Napoleontic dreams again. Furthermore they sought revenge for the lost Franco-Prussian war.
Germany: had the bad luck to be suddenly in 1871 the biggest and strongest country in Western and Central Europe. This made them naturally look suspicious in the eyes of the British (although they were mainly concerned about their naval presence). It also had the bad luck that Kaiser William 2 came to power in 1888. It's attitude towards Austria's politics was ambivalent: at first it encouraged Austria and, after seeing the Serb answer to the ultimatum of 25 July 1914 declared that there was no reason for war.
Great Britain: Could have made its intentions clear in the early days of the July-crisis, but didn't. Didn't care about Belgium, it was only the idea of Belgian harbours (and of course as the Dutch were pro-German in those days) the Dutch ports in German hands right before their nose. In fact, Britain could have stopped all of it if King George would have pressurized William 2 to stop Austria. But he did not. Also, British-German relations were easing up considerably in the early months of 1914.
Italy: played along for Southern Tyrolia. The Italian decision not to adhere to previous promises of joining Austria and Germany, but instead to join the allies was solely taken by one man, it's prime minister. In the Alps the Italians have never been able to win from the Austrians.
Russia: perhaps the biggest perpetrator. Seeking more influence to the West and a change to take revenge on Austria for the 1878 war. Nicolas could have stopped Russian involvement, but he was too weak to stop his generals and he was generally not liked by his ppl. It was the Russian mobilisation that set things in motion for WW1, what would have been normally a local Balkans-conflict.
Serbia: had been festering the atmosphere in Europe for over a decade at the time, all in search of their dream: the Pan-Slavic state. Politician had few sympathies for the Serbs, they were called King-killers as they killed their own king and the crown prince of Austria. While it had been messing around with its Slavic neighbours for a few years at the time, it now was messing with the wrong guy.
I leave Turkey out of this as the war against the Ottoman Empire has the mess we have in the Middle East nowadays as a consequense. Also, I leave Poland out of this as Poland at the time was not a country de facto: split by three other countries, it was not on the map at the time.