Serbia
Explanation:
Serbia, at the beginning of the 1990's, with Slobodan Milosevic as its leader, had a very aggressive and expansionist politics, unfortunately also manifested on the terrain as well. The Serbians wanted to get Bosnia and attach it to its territory, but Bosnia didn't just had Serbian population, but also Croatian and Muslim populations. The Serbians saw their military superiority over them as a chance to perform an ethnic cleansing, and so they did, attacking innocent civilians and killing thousands of Muslim Bosnians, as well as Croatians. Luckily, the international community got involved as well, so the massacre was stopped, though there was lot of damage already done.