During the second Punic war nearly half of the republic defected, and almost all Roman allies turned on them. Including a pro-Carthaginian uprising in Sardinia, and and outright defection of Syracuse to Carthage.
I guess you’re free to interpret things however you want, but “cohesive and loyal” is such an incredibly creative interpretation of the Roman republic during Hannibal’s invasion.
I believe you are incorrect about the defections as well. One source supporting this notion is Adrian Goldsworthy's "The Fall of Carthage: The Punic Wars 265–146 BC", which, by way of Wikipedia, claims:
https://en.wikipedia.org/wiki/Second_Punic_War#Hannibal_cros...
However, the majority of Rome's allies remained loyal, including many in southern Italy.
> Several of the city states in southern Italy allied themselves with Hannibal, or were captured when pro-Carthaginian factions betrayed their defences. These included the large city of Capua and the major port city of Tarentum (modern Taranto). Two of the major Samnite tribes also joined the Carthaginian cause. By 214 BC the bulk of southern Italy had turned against Rome.
The fractious nature of Senate politics was a consequence of a culture and set of institutions that encouraged and allowed domestic conflicts to be settled within the confines of established political processes, so that the Senate as an institution, while often beset by bitter internal political strife, did not face challenges from external centers of power within Roman society. The power and resiliency that this gave the Senate allowed it to maintain control even when Rome was under extreme stress, like it was in its depopulated state during the Second Punic War when it managed to raise two dozen legions.
>>The only groups that remained loyal to Rome during the second Punic war were the northern Latin cities.
You're correct that there was a lot of defection, but most cities in the Italian peninsula remained loyal to Rome.