The Union Forever
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This time, they try to expand from the Rus area (mediaeval Russians, originally under the yoke of corrupt church and the Boyars) into Roum (ancient Romans), who still use slaves for manual labour and are against the American way of freedom for all.
There are various traitors within all camps, from people who may lose their power base to ones who aren't too happy at being stuck on an alien world.
The main battles are fought at sea, with the newcomers introducing ironclads alongside Roman galleys. Luckily the enemy seem to give the Yankees time to build a fleet of them, which manages to swing the tide of battle.
A decent book, but all the outcomes seem a bit too pat, so you're never really too worried if the good guys will manage to come through.