1st Punic war
Trouble with the Mamertines
Agathokles king of Syracuse, had when he had no longer need for them, dismissed a group of mercenaries. These men originally coming from Calabria called themselves Mamertines, after the Romans, sons of Mars. These men went to Messana (Messina), where they were received in friendship, but soon they showed their true faces. They removed or killed anyone they didn't like and made the women their slaves. Now they had control over the city they turned the city into a pirates nest.
A cry for help
When they made too much trouble, the new and young king Hiero of Syracusae, marched with his army to Messana and drove the Mamertines back into the city. Now they asked the Carthaginians for help and when given they let them enter the city.
Roman counter measures
The Romans, on learning this, were all but happy; Carthago a great power virtually on their doorstep and when Messana fell, the whole island would soon be theirs. Though this is not found in the ancient books, it is more than likely the Romans induced other Mamertines to ask them for help. Even Polybius acknowledges the Romans had no just war, but from their military point of view they had no choice and marched to Reghium and crossed the straits of Messana.
For some reason or another the Carthaginian commander was persuaded or threatened to leave the citadel and the city. His men thought this to be a bad move and in good Carthaginian tradition they crucified the man. But if they were against leaving the city that much, why didn't they refuse to leave? But in the end they were camped next to Hiero's men, besieging the city.
According to Cassius Dio the Roman consul Appius Claudius tried to mediate between the two parties, but Polybius tells us he attacked immediately the Syracusans and when they had left during the night, he attacked the Carthaginian forces as well. When they withdrew also, he marched to Syracusae and started a siege.