Cuckoos

So, you want to take over a world. Are you sure? It's been done. No point reinventing the wheel.

Assuming that you have identified a suitable world, and are certain that it is right for you,

your first "tough choice" is going to be about the mess.

You're taking property away from those who already own the world.

They're not going to like it.

How will you handle their objections to being taken over?

Do you want to kill them, enslave them, infiltrate them, or assimilate them?

Does your choice depend on how easily duped your targets are? How docile? How useful? How many?

Will you pass yourself off as one of them, and destroy them from within?

(Cuckoo model. Convergence model. Sith.)

Will you conquer by force of arms and assimilate as far as pragmatic?

(Romans, Christians, Normans, European Empires)

Will you eradicate as many of them as possible, and settle the new land?

(War of The Worlds, Independence Day, Sauron. Conquistadors.)

Can you live side by side, making use of them?

(The Time Machine. Parasite model.)

Dhimmitude model

(Dhimmitude is a system of taxing aliens while encouraging aliens who wish to avoid taxation to convert.)

http://www.snopes.com/politics/medical/exemptions.asp

Beauty/Beast model

Most babies are irresistibly cute, even babies of another species. Presumably, one could infiltrate and conquer

through love of the alien as a baby until the adoptive parents are vulnerable to tyranny.

(Tribbles. Gremlins. Cuckoos again?)