What caused Bulgaria to enter World War 2?
Bulgaria entered ww2 because of the treaty it had with Russia.
Russia and Bulgaria were side by side and their treaty had not yet
ended. as a result, Bulgaria was to some extent forced to join the
war otherwise it would be under extreme threats from Russia
With the HOPE that it could restore its lost territory during Balkan wars and WW1. And to avoid San stefano treaty and Berlin treaty (Bulgaria thought these treaties were unjustified and imposed to it).
Bulgaria was a constitutional monarchy during most of World War II with Tsar Boris III ruling with a Prime Minister and a Parliament. Prime Minister Bogdan Filov controlled the country with the Tsar Boris. They declared Bulgaria to be a neutral nation. I have provided a link for you below if you want to read more about Bulgaria during the war.