Explanation: Not just a military food! Roman garum was fish sauce left to ferment, and was a condiment they put on everything. Bread, meat, other fish, salad, porridge, wine, fruit, everything. It was considered such an essential foodstuff that, like bread and wine, it was provided to slaves. There were varieties that were cheap as could be, like wine, and like wine, there were varieties that would cost a year's wages for a common laborer just to get a single container of.
Since it wasn't, y'know, boiled or anything, just left to ferment, it was a great way to get fish parasites. In fact, it's widely suspected to be a contributor to the spread of Mediterranean fish parasites to the Atlantic during the Roman era. Y i k e s
It sounds gross af, tbqh. Even the Romans acknowledged that the breath of someone who had just eaten garum was revolting, though, with one Roman love poet (sarcastically?) complimenting a man for continuing to pursue a woman after she had eaten six helpings of garum. But it must have had something going for it to be so widely popular - de gustibus non disputandum est!
More relevantly to military rations, Roman soldiers were extremely well-fed. It's been noted by some classicists that for all the innumerable complaints we have from ordinary Roman legionaries about their conditions - hard labor, pay, monetary deductions, not enough wine - food, perhaps uniquely amongst professional armies, does not pop up as an issue! Roman legionaries were issued a regular ration of grain (to be made into bread), meat, beans and lentils, cheese, salt, wine, vinegar, and olive oil; and probably a pretty substantial amount of it at that.