This is actually a common misconception - he had lots of support - only his other commanders completely failed - the Battle of Dertosa in 215 BC won by the Scipio brothers against Hasdrubal Barca's army he was bringing to Italy really dealt Hannibal's efforts a terrible blow. Not only did he lose a large contingent of men coming to support him from Spain, but massive reinforcements from Africa were sent to replace those men lost to the Scipios to defend Spain. At such a critical time, with Rome on the back foot, this could well have been what Hannibal needed to win the war... but of course, some things were not in Hannibal's control.
They did manage to send him some reinforcements in 215 BC, but - possibly under Hannibal's orders - other reinforcements were sent to other war zones - an army was sent to try and take Sardinia, but failed, and the army sent to Sicily was ultimately destroyed by an epidemic, though Hannibal had tried to support this push by sending one of his best officers, Muttines, there with some Numidians. The Carthaginian navy was poor, and even when they outnumbered Roman fleets they either sailed away or got defeated. In Italy on two occasions large numbers of men coming to reinforce him from his allies were completely destroyed.
Towards the end, Mago was still sent to purchase mercenaries among the Ligurians and Balearic Islands which he used to hassle the Romans in northern Italy in the last years of the war, before being recalled.
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