How would they do it?

Probably use of promovement propraganda, and martial law.

But propaganda doesn't make people pay taxes.

A marza in geostationary orbit around your capital city would make you pay taxes!

On a more serious note I disagree with BrotherBulger. Propaganda can make people do almost anything and believe almost anything if it is prevalent enough see "1984" the book by Orwell.

Edit: Especially the bit about kids reporting their own parents to the authorities for thought crimes.

Another Edit: there are real world examples of the power of propaganda as well especially during the anti communist "witch hunts" in the infamous Joseph McCarthy era in the states and many examples in the USSR.

Human thinking is hardly infallible. What has been done in the past is prisoners of war would be asked to make copies of documents by hand, in recent cases, those documents promoted Communism. Even though the POWs were never asked to believe what they were reading and rewriting, they eventually started thinking along the same lines as those documents and when they were repatriated, they were going about talking about whatever they saw. It only takes exposure to do so.

And thus it all goes back to promovement propaganda and martial law. LOL

Think a pro-democratic government plus  "homeland security" and "patriot act"..the FEAR of Vasari invasion (Independence Day)  and Advent forced conversion (the Ori) would make for some pretty loyal citizenry.

 Well, how would the Vasari gain loyal citizens on their conquered planets if they enslaved all the native sentients on the surface?

Loyalty is not needed. Only obedience.

But the longevity technology would be a pretty neat carrot, as such.

If the TEC controled settlements by use of promovement propaganda and martial law, how would they gain citizens if they destroyed the colony's defense fleet and then sent down an occupation force?

Natural births?

 You know when you conquer a planet you have to upgrade the Civilian Infrastructure (which is basically building advanced surface cities along with infrastructure such as: transportation network roads/rails/tunnels, energy creation and distribution facilities, communications networks, housing/shelter, manufacturing plants, and water and waste management) Why would building cities make people pay taxes?

People have needs, even alien citizens. Sales tax on food, clothing, etc. And people are already paying some taxes, then they need jobs to provide themselves and their families with these basic needs, and thus that income is taxed.... that's how.

 XxTaLoNxX, I would think that orbital bombardment targets and destroys Government facilities,therefore creating chaos and anarchy across the planet and forcing most of the civilians into refugee camps so I doubt they would be paying taxes while they are without adequate food,water,sanitary conditions,shelter,power,and medical care.

I think I may have inadvertently changed the topic.

Exactly why the game has underdevelopment taxes in the game. You see you would have to provide for these refuge camps and just ask FEMA, this is a very expensive alternative to a properly built up and working infrastructure. Although once there is an infrastucture in place as well as a working government then you would start to see a buildup of jobs, trade, and increased income for both citizens and government.

Why would you have to upgrade Civilian Infrastructure on formally neutral planets that already have a Sentient civilization,complete with large cities and established Governments on the surface?

Good question. The only ways I could explain that is that the game's mechanics are very inflexible, meaning that you just HAVE to upgrade the civilian infrastructure to avoid the underdevelopment tax. In a real world situation though, I would suppose that you would still have to build certain government facilities that the current planet would not already have present. No one wants taxation without representation... right?

This is kind of continued from my previous question, why would formally neutral planets with an already existing Sentient civilization have an Underdevelopment Tax?

Well you would still have to invest in optimizing what is already present so that you can take advantage of the existing population and the existing infrastructure. Imagine if Iraq was suddenly a state in the United States, we would definitely have to spend billions of dollars to integrate and optimize the country into US standards. With that example I think you can definitely understand if you use your imagination.

How exactly would the Full Terran Lockdown research increase the population cap?

LOL I think you are suspending the element of immersion and overanalyzing specifics like the tech naming now.

I am and I think I'm going to go meditate now.

Metta to you all,my brothers and sisters and happy holidays

  Why would you construct Civilian Infrastructure on Dead Asteroids when there are no civilians there to be taxed?

Like I said the game's mechanics are very inflexible. In a real world situation you wouldn't, but what kills me about Dead Asteriods is that there is not even 10 citizens able to chill in a command center. Basically they are only used for a tactical zone.

XxTaLoNxX, what do you think the population represents and is measured in?