Obviously this is easiest to accomplish on the Easy difficulty.
Since most, if not all planets, will have more than one continent, you will need to use ships to defeat one city on each of the other continents, then from that city build land vehicles to defeat the other cities on that continent.
This also can be done if you get the Faction superpower, and have NOT built any air vehicles.
Also you can forge an alliance with the superior nations on other continents. When they ask to merge, the continents are yours.
If possible, you can capture all of the sea cities that you can reach, and then when you have 6 cities, try using either ICBM if you are military, Global Merger if you are economic, and Fanatical Uprising if you are religious.
Don't create an air vehicle in the editor, as this may tempt you or you may accidentally buy one.