Insurrection is the thing you should fear most in the game. It is the ultimate ability of a spy, in that it will allow a spy to steal an entire colony from you. Don't worry, it isn't easy as long as you try to defend yourself you shouldn't have to worry too much. Here are the things you need to know. A spy cannot incite insurrection:

• on your capitol.
• unless your planet has negative happiness.
• on your planet if the controlling empire is more than 100 ranks above you in the empire stats.
• if he is controlled by an empire in your alliance.

Therefore you can very easily protect against insurrection by just keeping all of your planets insanely happy. How much happiness do you need? You can never have too much!

If you happen to want to steal a colony from another player, here's how you go about it.

1. Get the target planet into negative happiness. You can do this a number of ways but perhaps the two easiest are the "Incite Rebellion" spy assignment, and sending loads of Bleeders and Scows at the planet.
2. To ensure success the planet must be as negative in happiness your next colony cost is positive. So if your next colony would cost you 5,000,000 happiness, and you are able to get the colony to -5,000,000 happiness, then you would have a 100% chance of causing an insurrection when your spy is successful.
3. Get rid of all the enemy spies on the planet. You DO NOT want to fail this mission.
4. Get all your spies and your allies spies, but one off of the planet. If an enemy spy stops your insurrection there is a chance that a good number of allied spies will be captured and put in jail.
5. Run the insurrection mission and cross your fingers!

There are a few other things you should be aware of.

First, you need not get the entire -5,000,000 in the case above. For example, if you get -1,000,000 that's 20% of what you need to get your next colony, which means you'll have a 20% chance of success. If you fail on this chance you don't have to worry about your spies winding up in jail. This just means that you didn't have the planet upset enough with their government. 

Second, if you should be so lucky as to cause an insurrection, you will own the planet. It will get 10% of it's happiness back, but it will still be very unhappy. And if you didn't eliminate all the enemy spies on the planet, they will still be there just waiting to get the planet back from you. So be prepared for an up-hill struggle. It's not all roses after you win.