The history of the Balkan countries is interesting and everyone should be familiar with the events, past and present. To understand what happened in the Balkans during WWII requires knowing something of the history of their northern neighbors, Austria-Hungary and Romania, too. Make no mistake, these histories are confusing and often hard to follow. Without going into too much detail, here's a short summary.
The Balkans usually refers to all the land south of the Balkan Mountains which stretches east-west across the peninsula from the Black Sea (Bulgaria) to the tip of the Adriatic Sea (Slovenia). But often the borders and boundaries of Balkan states changed so most historians say the Balkans are those lands held by the Ottoman Empire since 1699.
While under Ottoman rule these lands were governed as a single state called Rumelia (land of the Romans). Basically, it's the lands of Bulgaria, Greece, Albania, Montenegro and Serbia. To maintain control or these lands, the Ottoman Empire was forced into creating principalities, which meant creating an area ruled by a prince, usually some rich guy born and raised there who the people could identify with. However, these principalities were intended to be puppet governments. Over a period of time, the Austrians were able to conquer some of the Balkan lands in northwest along the Adriatic Sea. They too created principalities to control these lands.
Throughout Rumelia there were many different ethnic groups. Some of these groups did not like each other at all. They honestly hated each other, worse than the Hatfields and McCoys ever did.
By the time the 20th century began, the Ottoman Empire was known as the "Sick man of Europe" because their empire had declined almost to the point of collapse. Knowing the Ottomans were too weak to hold Rumelia, in 1912 the "Balkan League" declared war for their independence and won it within a few short months.
The Great Powers of Britain, Germany, Russia, Austria-Hungary, and Italy, took it upon themselves to determine Rumelia's future. A conference was held in London to form, recognize and set up governments for the new nations created from the old Rumelia. They, too, created new principalities and appointed the prince. The meeting ended on 30 May 1913 with the signing of the Treaty of London.
Some of the new states were happy with the new treaty, but others were not. Bulgaria thought they should have been given more land and attacked to get it. The Ottomans and Romanians quickly countered and soon the Bulgarians realized they had bit off more than they could chew and had to quit and settle. The new Treaty of Bucharest gave Bulgaria less land than they had before.
In some of the other new states civil wars quickly broke out as some of the ethnic groups tried to gain control. Other states quickly invaded their neighboring states in effort to increase their lands. With so many small wars going on at the same time conducted by many ethnic groups with varying agendas, it's difficult to make any general statements regarding the overall history of the Balkans, except it was chaotic.
In the northwest Balkan states ruled by the Austria-Hungary Empire, these recent actions got many of those people thinking seriously about their own independence. Rumors of civil wars were strong. The new Balkan state of Serbia began preparing for an invasion of Austria-Hungary's Serbia to liberate the Serbs living there. To the east, Romania was threatening to invade for lands they wanted, like Transylvania.
The Austria-Hungary Empire was not prepared for war and tried to win with strong talk and ultimatums. This led to the assassination of their Crown Prince Archduke Franz Ferdinand while he was visiting Sarajevo, Bosnia. He was killed by Serbian militants. World War One began as the Great Powers got involved over these Balkan disputes.
Bulgaria and the Ottoman Empire joined Germany and Austria-Hungary, known as the Central Powers, to fight the Allies. Serbia became the Balkan hot bed as they were invaded on two fronts by Austria-Hungary and Bulgaria. Italy, also an Allied power, used their troops to protect most of Albania and helped the other Allied Balkan states, like Serbia.
At war's end, the Allies decided what was to become of the Austria-Hungary Empire and the Balkan states. Austria and Hungary were split into two and all their foreign territories were either awarded to other Allied nations or were given independence. This certainly changed the map on both sides of the Balkan Mountains.
But just like the Treaty of London, not everyone was happy with the new arrangements. Again, ethnic groups began warring with each other and nations began invading one another to suit their own agendas. This continued until the end of World War Two.
During the first world war, Italy protected Albania from the Central Powers. After the war they continued protecting them as they were under threat of getting carved up by their neighbors. As a result, Albania became heavily dependent on them to the point they were nothing more than a puppet government. In April 1939, before WWII even began, Mussolini invaded Albania and put them under the Kingdom of Italy. He felt he needed it so he could control the mouth of the Adriatic Sea. After the war began, he launched his invasion on Greece from there.
Between the world wars, the Balkan states in the northwest became unified as one nation, Yugoslavia. The ruling prince of Serbia inherited the thrones of the other principalities and crowned himself king of Yugoslavia. Not all of his subjects were happy with this and more trouble began.
World War Two in Yugoslavia was unlike any war ever fought. This wasn't so much a war of the Allies against the Axis, but ethnic groups against ethnic groups. Many of these groups had their own to grind with their own self-serving objectives.
Basically, these ethnic groups merged into two opposing forces, the Partisans and the Chetniks. The Partisans were backed by the Allies while the Axis supported the Chetniks. As the old saying goes, "The enemy of my enemy is my friend" and many of these groups picked a side to align themselves with but only to give them an advantage over their rivals. Some even allied themselves with their rivals to combat a bigger threat, with the understanding that once that was over they would fight each other again. This war was not fought with soldiers against soldiers but rather citizens against citizens, including women and children. Entire villages were exterminated.
With the exception of Greece, all these Balkan states fell under the Soviet Union after the war, known as the Eastern Bloc states. They soon found out how strong Stalin's iron fist was and had to set aside their differences. But just as soon as the Soviet Union fell apart and their troops left during the 1990s, these ethnic groups in Yugoslavia picked up right where they left off during WWII. It was if the USSR had never been there.
It took outside peacekeepers, like the US, UN, NATO, etc, to come in and stop the madness. Yugoslavia was disbanded and new small states were recognized. Today the peace continues but it hangs by a thread. Credit this to the former Yugoslavians themselves, as they are working hard to maintain that peace.