thumb|Coin minted under Mamertine rule, depicting a warrior
The Mamertines (, "sons of Mars", ) were Greek mercenaries of Italic origin who had been hired from their home in Campania by Agathocles (361–289 BC), Tyrant of Syracuse and self-proclaimed King of Sicily. After Syracuse lost the Seventh Sicilian War, the Greek city of Messina was ceded to Carthage in 307 BC. When Agathocles died in 289 BC it left many of his mercenaries idle and unemployed in Sicily. Most of them returned home but some, liking the climate and the prospect of adventure on a foreign island, remained. They played a major role in the lead-up to the First Punic War.
In 280 BC, the Syracusans appealed to King Pyrrhus of Epirus for help against the Mamertines.
Capture of Messina
The then-small band of desperados came across the walled Greek settlement of Messina built on a strategic location on the north-eastern tip of Sicily on the strait between Sicily and Italy. Together with the fort Rhegium on the toe of Italy, it was the crossing point between Italy and Sicily. Being a peaceful people, the inhabitants allowed the travelling mercenaries into their homes. After a time, the mercenaries became restless and plotted to capture the town. One night, the mercenaries betrayed their hosts and killed most of the population, who were unprepared. In this way, they claimed Messina for themselves in 288 BC. The surviving Messinians were thrown out and the property and women divided. After their victory, the mercenaries named themselves the Mamertines after the Oscan war-god Mamers.
Dominion over northeastern Sicily
The Mamertines held the town of Messina for over 20 years. They changed it from a bustling town of farmers and traders to a raiding base. The Mamertines became pirates on land and sea. Taking advantage of the war-weary Sicilians, they looted the nearby settlements and captured trade ships on the strait, carrying their plunder back to their base. They captured prisoners and demanded tribute. During this period, they struck coins featuring their name and images of their gods and goddesses. Their exploits made them rich and powerful. They began travelling further inland, even as far as Gela, and demanding tribute.