Explanation: The Roman Republic had a strange capacity to raise troops seemingly out of thin air whenever they suffered massive defeats. One historian of the time compared the Republic's capacity to raise new armies to 'water gushing from a fountain.'

The real reasons for this are multifaceted, but in the briefest sense, could be considered the core values of the Roman Republic.

Romans believed that they were born as a city of exiles and outsiders, and while they could be immensely chauvinist as to whose ways were better, this also meant that they were generally willing to accept and assimilate outsiders who 'wanted in'. This seems unexceptional to our eyes, but in antiquity, where many city-states had populations of 'foreigners' who had resided in the city for generations but were never granted the opportunity to become 'real' citizens, this was an exceptional advantage. So if there was a shortage of Romans all of a sudden, a little bending of the usual rules to 'create' new ones was very much in the cards!

The Romans also believed that their polity was for the good of the people. Again, this sounds unexceptional to our ears, but in antiquity, was positively radical. Not only did the Romans envision their Res Publica as working for the good of non-citizens (if without their input, conveniently) as well as citizens, but also, the Roman citizen body was much broader than most contemporary regimes. Athens, for example, the shining model of Classical democracy, only about ~10% of the city's population were citizens. In the Roman Republic, it's generally considered that over 50% of the city was made of citizens. For this reason, a very large percentage of people felt that they had a real stake in the survival of their Res Publica - part of the government, not just ruled by it.

And another was the aforementioned chauvinism. While not an unusual value in societies of antiquity, Roman pride and arrogance was noted as particularly intense even by contemporaries, and nothing stings quite like wounded pride. What, are you going to let a bunch of OUTSIDERS humiliate OUR Republic, citizen!? GET OUT THERE AND TEACH THOSE BASTARDS A LESSON THEY'LL NEVER FORGET!