system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the mercenary economy work

for example, during the punic wars, Carthage used a mercenary force as their army. From what I've read, campaigns could last years. How were these soldiers expecting to get paid and then spend their money if their period of service was so long?
gpt
They would be paid regularly. As the campaign progressed, the general could coerce the governments of lands they passed through to pay tribute. Non payment would result in pillaging.

The revenues from either would be used to pay/feed troops. If the pay stops the mercs typically abandoned the army quickly. 

Also one side could bribe mercenaries from the other side to not fight, or switch allegiances.