Cleomenes, king of the Lacedaemonians, surprised Megalopolis by night, forced the guards, broke in, and seized the market-place.

Awhile after, king Antigonus coming down to succor the Achaeans, they marched with their united forces against Cleomenes; who, having seized the avenues, lay advantageously posted on the hills of Sellasia. Antigonus drew up close by him, with a resolution to force him in his strength. Philopoemen, with his citizens, was that day placed among the horse, next to the Illyrian foot, a numerous body of bold fighters, who completed the line of battle, forming, together with the Achaeans, the reserve. Their orders were to keep their ground, and not engage till they should see a red coat lifted up on the point of a spear from the other wing, where the king fought in person. The Achaeans obeyed their order and stood fast; but the Illyrians were led on by their commanders to the attack. Euclidas, the brother of Cleomenes; seeing the foot thus severed from the horse, detached the best of his light-armed men, commanding them to wheel about and charge the unprotected Illyrians in the rear. This charge put things into confusion, and Philopoemen, considering that those light-armed men could be easily repelled, went first to the king's officers to make them sensible of what the occasion required. But when they did not mind what he said, slighting him as a hare-brained fellow (as indeed he was not yet of any repute sufficient to give credit to a proposal of such importance). he charged with his own citizens, and at the first encounter disordered, and soon after put the troops to flight with great slaughter. Then, to encourage the king's army further, to bring them all upon the enemy while he was in confusion, he quitted his horse, and fighting with extreme difficulty in his heavy horseman's dress, in rough, uneven ground, full of water-courses and hollows, had both his thighs struck through with a thonged javelin. It was thrown with great force, so that the head came out on the other side, and made a severe though not a mortal wound. There he stood awhile, as if he had been shackled, unable to move. The fastening which joined the thong to the javelin made it difficult to get it drawn out, nor would anybody about him venture to do it. But the fight being now at the hottest, and likely to be quickly decided, he was transported with the desire of partaking in it, and struggled and strained so violently, setting one leg forward, the other back, that at last he broke the shaft in two, and thus got the pieces pulled out. Being in this manner set at liberty he caught up his sword, and running through the midst of those who were fighting in the first ranks, animated his men, and set them afire with emulation. Antigonus, after the victory, asked the Macedonians, to try them, how it happened that the cavalry had charged without orders before the signal? and when they answered that they were forced to it against their wills by a young man of Megalopolis, who had fallen in before it was time, Antigonus replied, smiling, "That young man acted like an experienced commander."