Defend your cities until you get flight, then, once then, get 1 city (unless you already have) and hurry production (only if it is paid labor) of an airport. Build airports in your major cities and start massing troops over in the other spot and take city by city. Take about 5 (while going for motorized transport) cities, and declare peace (don't take small cities... yet). Send over some defensive units to defend your newly taken cities, and then declare war again. Take as many cities as you can handle with a good amount of offensive units. Declare peace and now take small cities. If you can reach them, send forces to defend them. If not, raze them. Lastly, but ONLY if you don't care about how people think of you, or if you need a ROP with some of your other allies/enemys, but force the county you warred with to give you a ROP. Send over you full hp forces on anywhere from 3-10 of their cities, depending on the defending unit, if riflemen (if infantry, i wouldn't even try with tanks, only modern armor would work efficiently), do 3, if spearman (that is only if they have not yet produced/gotten nationalism yet, then you can do 10. (only if forces allow it) Keep airlifting over forces to that continent. You should be able to take over any civ that way, but i don't know how close your are or if they would be able to hold you off.
PS-if you fail at any of these steps, don't move on, but wait a bit, and refit your forces and try the step again.