In 280 BC, Pyrrhus embarked on a campaign in Magna Graecia (i.e. southern Italy) against the Roman Republic known as the Pyrrhic War, followed by his invasion of Sicily. Ptolemy Keraunos secured his position on the Macedonian throne by giving Pyrrhus five thousand soldiers and twenty war elephants for this endeavor. Pyrrhus returned to Epirus in 275BC after the ultimate failure of both campaigns, which contributed to the rise of Rome because Greek cities in southern Italy such as Tarentum now became Roman allies. Pyrrhus invaded Macedonia in 274BC, defeating the largely mercenary army of AntigonusII at the 274BC Battle of Aous and driving him out of Macedonia, forcing him to seek refuge with his naval fleet in the Aegean. Pyrrhus lost much of his support among the Macedonians in 273BC when his unruly Gallic mercenaries plundered the royal cemetery of Aigai. Pyrrhus pursued AntigonusII in the Peloponnese, yet AntigonusII was ultimately able to recapture Macedonia. Pyrrhus was killed while besieging Argos in 272BC, allowing AntigonusII to reclaim the rest of Greece. He then restored the Argead dynastic graves at Aigai and annexed the Kingdom of Paeonia. The Aetolian League hampered AntigonusII's control over central Greece, and the formation of the Achaean League in 251BC pushed Macedonian forces out of much of the Peloponnese and at times incorporated Athens and Sparta. While the Seleucid Empire aligned with Antigonid Macedonia against Ptolemaic Egypt during the Syrian Wars, the Ptolemaic navy heavily disrupted AntigonusII's efforts to control mainland Greece. With the aid of the Ptolemaic navy, the Athenian statesman Chremonides led a revolt against Macedonian authority known as the Chremonidean War (267–261BC). By 265BC, Athens was surrounded and besieged by AntigonusII's forces, and a Ptolemaic fleet was defeated in the Battle of Cos. Athens finally surrendered in 261BC. After Macedonia formed an alliance with the Seleucid ruler Antiochus II, a peace settlement between AntigonusII and Ptolemy II Philadelphus of Egypt was finally struck in 255BC. In 251 BC, Aratus of Sicyon led a rebellion against AntigonusII, and in 250BC, PtolemyII declared his support for the self-proclaimed King Alexander of Corinth.