First, you build a city nearby an enemy country you think you can defeat. Have workers completely surround the city with fortresses and then put 2 of your best defensive units (pikeman, spearman, musketman etc.) in each fortress, and 3 in the city. Then you declare war. They will waste all their offense trying to take the city. Unless you are way behind in technology, they won't even be able to wound your guys because of the fortresses. While defending it, have every other city keep building you best offensive unit (like cavalry, tank, modern armor etc.). I suggest you should make about 15. After their offense is spent, invade their cities like crazy without having to worry about offensive units intercepting you.