August 28, 1943: Bulgarian King Boris III dies under mysterious circumstances.
Shortly after returning to Sofia, Boris died of apparent heart failure. Many believed that he was poisoned by Hitler in an attempt to put a more obedient government in place. But this seems highly unlikely, since his death came several days after his return. The evening before the illness occurred, Boris had an official dinner in the Italian embassy. Others suggest that this was a Communist plot to destabilize the monarchy. Source: Boris III of Bulgaria
While visiting Hitler in Berlin, he agreed to declare war on Great Britain and the United States, but not on Russia. His mysterious death, possibly by heart attack or by assassination, soon followed. Source: Boris III. The Columbia Encyclopedia, Sixth Edition. 2001-05
What's the source you are using that he was assassinated?