lol. I hate when that happens. You've allocated all your military resources into a war with a neighbouring faction, or annexing a few regions, and BAM! Another neighbour goes to war with you and you haven't quite yet developed enough to be able to fight substantially on two fronts.
This is my Carthaginian Campaign in H/H. The year is 234BC.
I started this campaign by removing the already installed systems of Government in Southern Iberia, and replacing them as allied kingdoms so I could just recruit client rulers there and not have to worry about public order, given their distance from Kart-Hadast. I also took the general from Mastia, and Lilibeo, so that I could put them into my main army out of Carthage, and just use a heap of Generals Bodyguard units in lieu of sacred band cavalry which I couldn't recruit yet. Given that I'd used Poeini Citizen Militia in the early stages of the M/M campaign I posted above earlier, I knew how terrible they were, even for Medium difficulty, so there was no way I was using them as anything other than garrison troops in a H/H campaign. I disbanded all of them, save for one unit each in a few settlements, and spent the rest of my money creating Libyan Spearmen and Liby-Phoenecian infantry.
Initially this was a fairly expensive move to make, but later on really didn't matter, as now that Mauritania, and the rest of North-West Africa are all allied kingdoms under my faction. Having so many client kingdoms with allied/client generals meant that I could spend less time developing buildings for public order (temples, garrison posts, other monumnets, etc), and start them off by strongly developing the local economic infrastructure with roads, mines, farms, trading ports, etc. The two factions in southern Iberia (Gader and Mastia I believe) both have fully developed gold veins/mines, as well as all of North-West Africa, Alalia and I think maybe Lepki, too, if you can build them there.
Once I'd finished doing this I was absurdley rich, and the Romans had already taken Cisalpine Gaul and were expanding into Transalpine Gaul and the Dalmatian coastline. They also had a strong economy. I decided it was time to stop them before they grew too far and numerous for me, so I took my experienced army and retrained it in Kart-Hadast (I could recruit sacred band cavalry at this point, so I replaced all the generals in this army save for the faction leader), and sent him to Sicily to beseige Syracuse while I created a second army.
Using the second army I sent it to Sicily to beseige Messana. I waited the seige out on both cities, as I wanted to save as many as possible for when I did run into the Romans eventually. I did much the same with Rhegion. Once Sicily and the southern tip of Italy were firmly under allied kingdoms, I moved for Taras and Capua. Taras was very poorly defended, so my King's/Faction leader's army took it quite easily, while I beseiged Capua hoping to lure the Romans into an open field battle, as this army had about five units of Elite African Phalanx with some sacred band infantry, elite Liby-Phonecian infantry and Sacred band cavalry, which proved to be a powerful combination for pinning and flanking maneuvers. While doing this, I created two more armies, and bolstered one of the slightly depleted armies in Italy with mercenary heavy Samnite swordsmen.
I now had four full sized armies comprised of various types of heavy infantry, spearmen, phalanx troops and heavy cavalry, but was still making about 11,000 mnai per turn. It was obvious to me that spending so much time developing trade and production amongst my colonies was really quite literally paying off.
That pretty much brings it up to where I am now. I've taken Arrettium, Bononia, Ariminum and I've beseiged Segesta.
When I beseiged, then captured Ariminum, it was quite well defended, and I subsequently lost about 1/3-3/7 of that Army, which I'll soon either bolster with reinforcements and mercenaries, or just merge with another army in the region.