Complex sentence: Madero had appointed Huerta as army commander when he first achieved power, but the general had turned against him after a stalemate between Madero's forces and those of Félix Díaz, a nephew of the former president.
Split into simpler sentences: Madero had appointed Huerta as army commander when he first achieved power. Huerta had turned against him after a stalemate between Madero's forces and those of Félix Díaz, a nephew of the former president.