I came across a Web site for re-enactors of the Roman Legions, which is quite interesting to military history buffs. Among the articles it contains is this one on the bread made by the legionaries - and how to make it yourself today.
Although known since the 5th millennium BC, there are evidence that bread start to become a very popular form of food in Rome only at the end of the Second Punic War. It is indeed in quite a short period of 25 years, between -200 and -175 that the first public bakeries (fifty) make their appearance in Rome. At the same time Cato the elder, who has also participated in a victorious military campaign in Spain, explains how to bake bread under a "clibanus"(pottery in the shape of a bell). Moreover, we find evidence proving that at that time both Romans and Carthaginians used clibanus as portable ovens like the type described by Cato. Etruscan models dated back to the 5th century BC show that this type of portable oven was already used in Italy for a long time. Ideally suited for baking military bread, it is also very easy to transport and use. The clibanus is thus the ideal instrument to cook bread in campaign.
There's more at the link, including lots of photographs illustrating legionary grinding-stones, the 'clibanus', and how the soldiers prepared their bread. This is what the finished product looks like.
It looks very similar to what we called 'camp bread' in Africa, cooking it over fire-heated stones. I'm on a low-carb diet at the moment, so I can't try that right away: but soon . . .