What caused Bulgaria to enter World War 1?
Because they were not happy with the result after the balkan wars. After the first balkan war, the countries that participated earned accupied lands. Everyone else was happy, except Bulgaria. They complained that they wanted more and more, trying to make Bulgaria how it looked like 700 years ago. Then after the second balkan war which bulgaria lost, they lost their lands (thrace and tiny bits in western Bulgaria Boseligrad and Dimitrovgrad). Bulgaria wanted revenge, paticularly to Serbs. Then they joined the Germans, Austrians and Hungarians to attack Serbia from behind. Lets face it, every country even Switzerland wants power and look where that lead us.