In the summer of 479 BC, the Greeks got together a huge army by the standards of the day, and marched out of the Peloponnesus. The Persians retreated to Boeotia, and built a fortified camp near Plataea. The Greeks refused to be drawn into the open cavalry ground near the Persian camp, and this led to a stalemate which lasted eleven days. When their supply lines were disrupted, the Greeks retreated a distance, and their battle-line fragmented. Mardonius thought the Greeks were in full retreat, and ordered his men to pursue them. The Greeks (particularly the Spartans, Tegeans and Athenians) turned and gave battle, routing the lightly armed Persian infantry and killing Mardonius.