N’DJAMENA, Chad—Chadian President Idriss Deby Itno, who ruled the central African nation for more than three decades, was killed Tuesday on the battlefield in a fight against rebels, the military announced on national television and radio. The military said that a transitional council will be led by Deby’s son, Mahamat Idriss Deby Itno. The stunning announcement came just hours after electoral officials had declared Deby the winner of the April 11 presidential election, paving the way for him to stay in power for six more years. The circumstances of Deby’s death could not immediately be independently confirmed due to the remote location of the battlefield. It was not known why the president would have visited the frontlines in northern Chad or participated in ongoing clashes with the rebels who opposed his rule. Deby, a former army commander-in-chief, first came to power in 1990 when his rebel forces overthrew then-President Hissene …