when war is brought to your homeland, it is very sudden and with an increased difficulty, the attack is deadlier. the enemy will send stacks and stacks at you, without any breaks. There is hope though. The enemy will send lots of siege units. use units that flank units and kill siege units. send them and kill them. When those are gone, the enemy cant attack your cities easily. send siege units to damage the stack severely. then, send your biggest stack to kill the army. if the enemy wont negotiate, another stack is on its way. Kill those stacks or launch a counter offensive. If you are in the modern or future era, be prepared for nuclear strikes. Send workers to clear the damage ASAP. Airstrikes may be likely, and blocades are inevitable. The later the era, the worse the damge. stealth destroyers and submarines, unless dealt with quickly will result in a catastrophic loss of life in coastal cities. Destroy the stealthy menace quickly. this is how i would fight a defensive war. this could be a method for new players looking at this. if you have more tips for this topic you can post them here. thank you!