How do you counterfeit gold?
Try the link up there, there are lots of surviving examples of ancient counterfeit coins; there’s also been a recent uptick in the modern counterfeiting of ancient coins.
The Roman army was paid in vinegar and salt (the term “salary” comes from “salarium”; still, some claim it means gold), over and above room and board; actual cash was paid after big paydays, such as accompany a successful campaign. The biggest such payday was Trajan’s conquest of Dacia, which is sometimes pointed to as the economic peak of the Roman Empire.
During the near-century of crisis, the various usurpers, claimants, and pretenders pretty consistently used minted coin (or rather, the promise thereof) and pay increases (or rather, the promise thereof) to win support of troops. If they couldn’t come up with it for whatever reason, they tended to have short careers (the predecessor of Septimius Severus mentioned above had that very problem, wound up wondering where his palace staff had suddenly got to, and discovered what he was last to find out as Roman soldiers hacked him to death.
Diocletian ended the crisis by reinstating in-kind pay for the Roman army, wiped out the remaining rival ‘emperors’, divided the Empire, set up a system of succession, stiffened the borders, (re)expanded the Empire here and there, and after more than twenty years of rule, retired and forced his colleague to retire, thus testing his system of succession.
Rome had an understanding of currency that eludes a lot of people today; coin was invented in Lydia, by the King thereof, and it came to replace other systems of trade by providing a common, sanctioned medium of exchange with the imprimatur of the ruler right on it.