Contrary to the song, war’s actually good for a great deal of things in Imperator Rome, and waging battles with your armies will comprise a lot of your time in the classical era strategy sandbox. Although combat doesn’t play out in granular detail, it’s still an involving mechanic with plenty of subtleties which may make the difference between victory and defeat.
Every nation begins the game with at least some troops in Imperator Rome, but if you have grand designs for a vast territory well beyond your own current borders you’ll have to build a bigger army at some point.
This can be achieved in several ways. The quickest and easiest is to hire mercenaries, who you’ll see on your provinces ready to go. These are pre-baked armies ready to head straight into battle with no build time, simply requiring that you pay to hire them first. These are useful in a pinch, or when time’s a factor and you want to make a move swiftly on your enemy without waiting for troops to be recruited in your provinces and then sent to the front line. Mercenaries are less loyal to you than armies built in your own cities, though.
Next, you can use the macro builder at the top-left of the HUD to build troops. Just click on the province you’d like to build troops in to highlight it, then select the unit type and building will commence. This costs money just like recruiting mercenaries, but your troops will have more loyalty. Finally, you can head into each province’s management screen and recruit troops manually from there.
As with other Paradox strategy titles and the Total War series, combat in Imperator Rome can be likened to a particularly complex and historically rich game of rock, paper, scissors. Each unit type has strengths and weaknesses against the others, so victory is about matching your opponent’s armies with a force designed to counter them.
For example, cavalry units are particularly good at decimating archers - especially heavy cavalry. But heavy infantry are adept at wiping out cavalry - and so on. It pays to do some research on what your opponent’s army composition looks like before building a force of your own. And if all else fails, holding the numerical advantage goes a long way.
It’s also crucial to appoint a commander to your army - they’ll be destroyed in battle without one. Commanders have a massively positive impact on morale, so even if your losses are matching your opponent, you’ll likely still lose a battle without a commander because your morale will collapse sooner.
Starting A War
There’s a bit of diplomatic bureaucracy to undertake before you go marching into battle in Imperator Rome. You need a reason to invade enemy territory and have your troops cross borders, and the simplest way to do this is to fabricate a claim on someone else’s land. Once you do this, you’ll need to wait a few days for your diplomats to leave the affected region, then declare your Casus Belli (your case for going to war, in other words) and march your troops in.
Once that’s done, you start taking enemy territory simply by occupying it. When you have occupied all the territory you wish to take, you can now enter peace talks and negotiate that the land in question now belongs to you. Simple, eh?