Power = troops + equipment (tanks, aircraft, ships, missiles) + readiness + supply. Arms factories produce equipment, or you import it; soldier training comes from barracks and military schools. The Military tab shows the whole picture in one place.
Country card → Declare War. Declaring demands resources up front (about 30 days of supplies + fuel) and fighting starts after ~14 days of mobilization. If the Required Resources list is not all ✓, stock the army first.
At war the army burns food, ammunition, fuel and medical supplies daily (about 3× peacetime). If supply drops, your effective power collapses — an army that is big on paper but hungry loses. Fill the depots BEFORE the war.
Groups feed on occupied regions, poverty, unemployment, unrest and war; security services, the Counter-Terrorism policy, counter-intelligence and prosperity suppress them. Attacks send power plants, roads, mines and tourism into repair; cartels extort the treasury. From Military → Internal security you can launch an operation (money and a little approval) or declare an amnesty for separatists and insurgents. The lasting cure is to remove the roots: jobs, prosperity, calm. Remember that rivals can secretly fund a group; your intelligence reveals the backer.
Every region you annex in war stays occupied for a while. Tap the region and pick a policy: Moderate pays for a garrison and integrates slowly; Harsh brings requisition income and crushes uprisings but raises resistance, discontent and diplomatic cost; Integration needs investment but calms resistance and speeds up integration. At 100% integration the region becomes fully yours.
The character of whoever you appoint to the general staff and the service commands feeds straight into war: your strength, your losses, civilian casualties and what the front burns through. The military budget share is set here too.
Once research opens the producer of the newer class, you can modernize what's in your inventory for less than building it fresh. The batch sits in the yard while it's worked on — it doesn't count toward your strength — and joins the inventory as the new class when it's done.
With enough warheads, countries without them won't dare attack you. You can run the program in secret, but nobody knows about it so it deters nobody — and if it comes out you face treaty sanctions and a heavy hit to your relations.
Once the war has run its course, the Peace Table opens: the front line earns you points, and you spend them on demands (tribute, facilities, population, border regions, even the whole country); the chance of acceptance is shown. The other side can refuse — try again a little later. Lose a war and you may lose a border region yourself; when a war stalls, your enemy offers a white peace. After peace, a 5-year ceasefire lock applies.