You do need quite a bit of preparation, but that's half of the fun. Generally it's best to wait until flight and those types of tech to attack other continents. Carriers and destroyers can drop a city's defenses to nothing, marines can usually storm it on the same turn, then units can be airlifted in on the same turn. Once you've secured a foothold, you won't need your transports anymore because you'll be able to airlift units in faster than you could send them by transport. If you need to take a very heavily fortified position, you can just nuke it. By the time you get to this level of tech, production decides everything. Whoever can make the most nukes the fastest, the most tanks, the most marines, etc. is going to wipe out all the competition very quickly, and nearly bloodlessly.