Build a city close to an enemy country that you think you can defeat. Have workers completely surround the city with fortresses then put two of your best defensive units (pikeman, spearman, musketman, etc.) in each fortress, and three in the city. After that, declare war. They will waste all their offense trying to take the city. Unless you are very behind in technology, they will not even be able to wound your troops due to the fortresses. While defending it, have every other city keep building you best offensive unit (for example, cavalry, tank, modern armor, etc.). Making about fifteen is recommended. After their offense is spent, you can invade their cities without having to worry about offensive units intercepting you.