Here's as an answer I gave a while back as to Bulgaria's role in WWI.
In WWII, suffice to say it was similar to the first war. Bulgaria had wanted vengeance for the Balkan Wars, and WWII added vengeance for WWI to that list. By 1941, Germany was the dominant power in Europe, the states of the Balkans were falling in line with Germany and her allies, and there was pressure on the king from right wing nationalist opinion, to make good the humiliations of the past decades, and thus secure for Bulgaria a favourable place in the 'Neue Ordnung' (New Order) envisioned by Hitler.
In WWII, the Bulgarians supported the Germans and Italians in their invasions of Greece and Yugoslavia in 1941, and occupied parts of Greece and Yugoslavia that they had previously occupied in WWI. Bulgarian troops served as rear area troops on the Eastern Front; then, in 1944, along with Romania, Bulgaria quit the axis, allowing Red Army troops to enter the country and continue on with Bulgarian armies as support, to NEARLY trap and destroy Germany's forces in the Balkan peninsula.