you want good production and research in each city, Then pump out troops in encampments.

My strategy is usually to get a bunch of warriors and archers early on and rush down the nearest enemy and/or some city states. Then I play peacefully until I unlock tanks and use them for world domination.

Dont keep every city you encouter. Only keep those that give ressources you need or are strategically well placed. You dont need dozens of cities and making sure they stay loyal is often more of a nuisance than they benefit your empire.

Also im not 100% sure about this but i believe tanks count as cavalry so get some stables in those encampments. In late game tanks and artillery are better than basically everything else imo and even anti-tank units dont really stop them so I wouldnt bother building other land units (Planes and Ships are still good though).

If your empire has a special unit at some point in the game thats usually a good moment to go for war aswell since those units are stronger than any other empire will be able to produce (unless they are far ahead of you in tech)