So I don't really know what 'at war' means. In vb6 there are tick boxes for hostile, neutral, friendly.
In C#, I've looked all over the intelligence screen, all over the ground forces screen, colony summary. I've tried SM setting relations on both sides to -200. I've tried running the game for 10 years in case there was a hidden truce timer or something blocking things.
Well, a quick short cut with Aurora-controlled NPR is to start shooting their ships with your ships. That should get them hating you and shooting back in a hurry.
If both sides are player-controlled, the places to look for the ground combat controls is on the colony screen, and on the ground forces design screen. Check the tabs near the right end.