The Persian army seems to have made slow progress through Thrace and Macedon. News of the imminent Persian approach eventually reached Greece in August thanks to a Greek spy. At this time of year the Spartans, "de facto" military leaders of the alliance, were celebrating the festival of Carneia. During the Carneia, military activity was forbidden by Spartan law; the Spartans had arrived too late at the Battle of Marathon because of this requirement. It was also the time of the Olympic Games, and therefore the Olympic truce, and thus it would have been doubly sacrilegious for the whole Spartan army to march to war. On this occasion, the ephors decided the urgency was sufficiently great to justify an advance expedition to block the pass, under one of its kings, Leonidas I. Leonidas took with him the 300 men of the royal bodyguard, the "Hippeis". This expedition was to try to gather as many other Greek soldiers along the way as possible and to await the arrival of the main Spartan army.