The Balkans were known as the "Powder Keg of Europe" because of its constant disagreements regarding land distribution and an extreme sense of nationalism, which made the area a hot bed for small conflicts and larger wars. The Balkans had this title throughout the beginning of the 20th century, before World War I.
Before the start of World War I, many countries in Europe wanted control of the Balkans. Russia and Austria wanted Serbia and many European countries opposed the Ottoman Empire. The instability in the region eventually caused the First Balkan war in 1912. Bulgaria was unhappy with the treaty that was drafted to end the first war so that country began the Second Balkan War in 1913.
Germany and Russia were the two largest countries involved in the ongoing Balkan conflicts. They weren't always sure what to do because they had an obligation to their allies to assist if and when the situation warranted their intervention, but because they were on opposites sides, both Germany and Russia had to be careful not to anger the other country. At the time, neither country had any idea its involvement would help to cause a major world war, but in the end, that's exactly what happened. The fighting about the Balkans ultimately started World War I.