Complex sentence: Angered with his betrayal, in 924 Simeon sent an army led by Theodore Sigritsa and Marmais to crush the Serbs but the Bulgarians were insufficient in number, they were ambushed and defeated and the heads of their commanders sent to Constantinople.
Split into simpler sentences: Angered with his betrayal, in 924 Simeon sent an army led by Theodore Sigritsa and Marmais to crush the Serbs, but the Bulgarians were insufficient in number. They were ambushed and defeated, and the heads of their commanders were sent to Constantinople.