Treaty of Sèvres. After trying to obtain even more land in the Greco-Turkish war, the Greeks lost all of Asia Minor and the prize of Constantinople. In the 1970s the Greeks got greedy once again and launched a pro-Greek unification coup in Cyprus. Turkey responded by invading the island country. Today Cyprus is a divided nation paying the price for being geographical linked to two rivaling countries.
Serbia also went through a historical roller coaster. After starting World War I Serbia managed to gain many new lands by becoming the lead state in the union known as Yugoslavia. However, the state of artificial and only lasted because of the iron hand of Josip Broz Tito. After his death there was a short period of peace followed by a series of civil wars lasting until 1999. Today Serbia is a shell of its former self losing long time partner Montenegro and having its southern province of Kosovo being governed by the United Nations with Kosovo's final fate being unknown.
Romania has had a tough history but has come out more of a winner then the previous two. Transylvania was once part of Hungary but given to Romania because of Hungary's pro-Nazi government during World War II (Update: Romania receieved Transylvania in 1919 but Hungary reoccupied it in 1940. While Romania then adopted a pro-Axis agenda, in 1944 a coup changed Romania into the Allied camp) While the region has many Romanians who welcomed their homeland's reign many ethnic Hungarians still feel oppressed. Such is the condition of nations not in their own state. The country of Moldova originally hinted that it wanted to join Romania after Moldava's independence from the Soviet Union. However, the region of Transnistria has derailed this with an armed attempt to rejoin Russia. Plus, Moldova is too poor to be supported by growing but still cash-strapped Romania.
The last country on the map, Bulgaria, gained access to the Aegean Sea briefly but lost it to Greece during the Balkans Wars. Besides that Bulgaria has manged to have stable borders. A triumph considering the recent history of Europe.
The Balkans have been active the last 100 years just like the rest of Europe. So below is a parting video which I have featured before. It shows European boundary changes from the start of World War I until the mid-1990s.