He was the leader of the Bulgaria during WWI. He was known as the Tsar of Bulgaria (monarch) during World War I. When he lost the WWI, he gave up his throne at the end of the war to his son Boris III.
Bulgaria and central power
Bulgaria was the last major country to join the war on the side of the Central Powers in 1915. Bulgaria claimed land held by Serbia and was eager to invade Serbia as part of the war. Since Bulgaria was joined last, there isn't a lot of information about Bulgaria.