Cleonymus, the younger son of Cleomenes II., king of Sparta, and uncle of Areus I., was excluded from the throne on his father's death, 309 BC, in consequence of the general dislike inspired by his violent and tyrannical temper. In 303 BC, the Tarentines, being at war with the Romans and Lucanians, asked aid of Sparta, and requested that the command of the required succours might be given to Cleonymus. The request was granted, and Cleonymus crossed over to Italy with a considerable force, the mere display of which is said to have frightened the Lucanians into peace. Diodorus, who mentions this, says nothing of the effect of the Spartan expedition on the Romans, though it is pretty certain that they also concluded a treaty at this time with the Tarentines. (See Arnold, Hist. of Rome, vol. ii. p. 315.) According to some of the Roman annalists, Cleonymus was defeated and driven back to his ships by the consul, M. Aemilius; while others of them related that, Junius Bubulcus the dictator being sent against him, he withdrew from Italy to avoid a conflict. After this, abandoning a notion he had formed of freeing the Sicilians from the tyranny of Agathocles, he sailed up the Adriatic and made a piratical descent on the country of the Veneti; but he was defeated by the Patavians and obliged to sail away. He then seized and garrisoned Corcyra, from which he seems to have been soon expelled by Demetrius Poliorcetes. While, however, he still held it, he was recalled to Italy by intelligence of the revolt of the Tarentines and others whom he had reduced: but he was beaten off from the coast, and returned to Corcyra. Henceforth we hear no more of him till 272 BC, when he invited Pyrrhus to attempt the conquest of Sparta. (Diod. xx. 104, 105; Liv. x. 2; Strab. vi. p. 280; Paus. iii. 6; Plut. Agis, 3, Pyrrh. 26, &c.)
According to Strabo he or Cleombrotus founded the city Heraea
Pyrrhus came very near to reducing Macedonia entirely, but, being usually readier to do what came first to hand, he was prevented by Cleonymus. This Cleonymus, who persuaded Pyrrhus to abandon his Macedonian adventure and to go to the Peloponnesus, was a Lacedaemonian who led an hostile army into the Lacedaemonian territory for a reason which I will relate after giving the descent of Cleonymus...
Cleomenes had two sons, the elder being Acrotatus and the younger Cleonymus. Now Acrotatus died first; and when afterwards Cleomenes died, a claim to the throne was put forward by Areus son of Acrotatus, and Cleonymus took steps to induce Pyrrhus to enter the country. Before the battle of Leuctra the Lacedaemonians had suffered no disaster, so that they even refused to admit that they had yet been worsted in a land battle.