I'd make peace with the more powerful countries until modern military tech arrives. Then build up an air/land/sea force large enough to completely surround them all. Then simultaneously invade and completely crush them all. Then do the same to the push-over countries. Takes a long, long, time.
I was never able to play civ but that's how I would run age of empires. I would even build on "friendly" land so that I could build an army that would kill everything in sight as soon as I switched them to foe.
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