I am not an expert on WW2 or Bulgaria at all but recently Simeon II of Bulgaria, former Tsar of Bulgaria made several interview in various French radio stations and I listened to all of them.

Bulgaria's role during WW2 mostly is due to Tsar Boris III's politics.

Following Bulgaria's defeat in WW1 and the abdication of Ferdinand I, Boris III came at the head of a country in a disastrous situation, financially mostly and tore between left and right wings extremists. During the first years of his reign, Boris III stayed out of politics ( in general, far from always ) and focused on helping and improving the lives of his subjects ( mainly the poorest ). It gave him an incredible popularity to his people but in 1934, a political and military organization named the Zveno made a coup d'etat and planned to create a Bulgarian military republic. A year later, Boris III regained his power and almost took full command of the country.

Bulgaria was a feeble country but with a popular sovereign. Unfortunately, the country was surrounded by gigantic powers in the name of the Soviet Union, the now Nazi Germany and Western powers. Politically, Boris III hated both the totalitarianism of the USSR and Germany but received no helped at all from the West such as the UK or France who had better to do. Bulgaria quickly became economically dependant of Germany in terms of trade and vital resource supplies.

In 1939, the Second World War broke out to the great despair of Boris who decided to remain neutral until 1941. As a pacifist, Boris had no choice to accept Germany's terms when its troops gathered at Bulgaria's borders. No need to say that its military would have been crushed in a matter of weeks ( or less! ) by the German machine.

So Bulgaria agreed to join the Dreimächtepakt ( the Tripartite Pact ) but both Boris III and his people played the neutrality card. The Tsar invoked the russophile population of his country who would not want to fight the Russians alongside Germany ( they would most likely prefer to do the contrary! ). Also, since the ridiculously severe Treaty of Neuilly of 1919, Bulgaria's army was supposed to be defensive only and Boris also tried to play this card.

As you can imagine, a trial of strength began between Hitler and Boris III at the difference that one was at the head of a powerful country. After the crossing of Bulgaria by German troops in 1941 and their arrival in Greece to fight alongside Italian soldiers, Hitler gave back Bulgaria parts of its lands lost after the Treaty of Neuilly such as Thrace or Macedonia. Bulgarian troops entered these lands but were barely seen as an occupation force and had very little to do.

In December 1941, Boris III and his government declared war on the allies but refused to declare war on the USSR. His reasons were that he would not have to fight UK or US troops because of the geography but if Bulgaria was at war with the USSR, the country would risked to be destroyed by the Red Army ( history showed that he was wrong concerning the first affirmation )!

Hitler was not very amused and pleased by this " strange ally " especially when Boris III refused and forbid several hundreds young men to create a regiment to fight the USSR. In short, Boris did all he could to save his people from a lost war but that did not prevent parts of Bulgaria to be bombed by the allies at the end of the conflict.

Also, it's not really relevant to your question but another huge part Boris III played in the war was the refusal to deliver Bulgarian Jews to the Nazi machine. In 1940, several anti-Jews laws were voted and the Brannik, some sort of Hitlerjugend were created. Some 50,000 Jews were to be victims of these laws however, Bulgarian people were not antisemitic and it barely existed in Bulgaria. For a year, these laws were null until 1941.

In 1942, " the Jewish question " is asked to be solved in Bulgaria by Hitler. A SS is sent to supervise it and around 11,000 Jews are deported from the newly annexed territories of Macedonia and Thrace. But Boris III, the people and some members of the government and parliament were not very keen on the arrest and deportation of the Jews living in Bulgaria. Huge demonstrations in front of the Tsar's palace for the saving of the Jews showed Boris III the popular feeling for the Jewish question.

The furious Fuhrer asked for an explanation and the monarch said that he needed " his Jews " as a labour force. Indeed, they were forced to clean the streets, build roads and whatever but some 50,000 Jews avoided deportation thanks to Boris' role and the Bulgarian people.

In 1943, Germany suffered Stalingrad and Bulgaria saw the future course of the war. Boris III met ( maybe not in person but through messengers ) allied diplomats. Hitler heard of the rumours and called the Tsar for a meeting. It was not a courtesy meeting as Hitler recalled Boris III all he owed to Germany, mainly the preservation of Bulgarian lives as the only role its military played in the Eastern front was the envoy of a couple of supply convoys ( food and medicine if I understood well ). So the Fuhrer gave Boris the order to send his troops against the Red army. The Tsar refused and left Hitler. Nine days later, Boris III suffered of violent vomitings and died aged 49 despite being a perfectly healthy man.

You can imagine what was said following the death of the Tsar of Bulgaria, mostly that Hitler had poisoned him. Indeed, Boris was not the greatest allied for Germany and Hitler could very well have hoped for a more " helpful ally " in the name of his successor because his heir was in fact 6 years old and in need of a regency. This regency was occupied by Boris' brother the Prince Kiril, a general and the very pro-German minister Bogdan Filov. A more interesting Bulgarian leadership in the eyes of Hitler.

In 1944, a little more than a year after Boris' death, the USSR declared war on Bulgaria whose military could not do a thing against the Red Army. The communist party of Bulgaria seized power and overthrew the Prince regent who ended executed by the communists. The young King and successor of Boris III, Simeon II was exiled after three years of being King ( without ruling as he was a child ).

THis was a very long post, sorry for that. But Boris' role in WW2 and Bulgaria's are barely known while I think that Boris was a great man. Who knows what glorious monarch he could have been should he have reigned in a better period of history.

The interesting thing about Bulgaria in WW2 is that they did not declare war on the USSR. So unlike Germany's other allies (like Romania and Hungary) they did not send troops to fight in the east.

Bulgaria did declare war on the UK and US, but geography meant that they were not involved with any fighting against these nations.

With the German position in the Balkans collapsing, the USSR declared war on Bulgaria in September 1944 and quickly overran the country, (effectively) taking it out of the war.