Whom does Alexander the Great attack in this scene from the "Alexander Mosaic" found at the House of Faun in Pompeii?

The Alexander Mosaic, dating from circa 100 BC, is a Roman floor mosaic originally from the House of the Faun in Pompeii. It depicts a battle between the armies of Alexander the Great and Darius III of Persia. The original is preserved in the Naples National Archaeological Museum. The mosaic is believed to be a copy of an early 3rd-century BC Hellenistic painting.