Originally posted by Dynamo:
The war in Iraq brings up some interesting points given that our TL 9 army is attacking a TL7-8 Army in Iraq. How many TL 12 and 13 troops would you need to quell a TL 9 planetary revolt? How about six? Would your answer have been the same before the war?
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The benefits of High TL societies in war are obvious, boiling down to "We can hurt and kill you, but you can't hurt or kill us". This can be accomplished by distance (from Air/rafts to orbital strikes), armour, weaponry ("smart" bullets, high rates of fire, etc), superiors information (orbital cameras, excellent traffic analysis, etc), and superiour information processing technology.
If the high-tech society is willing to take casualities and spend the cash, they can certainly crush any low-tech enemy.
I doubt that you could use just six soldiers to crush any major rebellion of a high-pop/low-TL world - especially if the population is physically dispersed on the globe. There is an exception: if the soldiers are actually sailors in orbit, with an ample amount of nuclear/antimatter firepower they are willing to use. Simply due to the physical scale of a planet, you will need at least a few thousand for most situations. Because the Imperium does not use warbots, increase the numbers by a magnitude or more.
If the planet is heavily populated (10 million+) and they have a habitable world (free air, water, fertile soil, edible local life), again increase the number of troops by at least a few factors, and possiblily by magnitudes. If the locals are willing to pay very high prices (say, 10,000 dead locals per Imperial soldier) you'll have to decide just how much people you are willing to kill (the Imperial Navy can always just glass over a planet), how sigificant the world is, and if you are comfortable with the political consequences of large-scale genocide.
Problems for High TL societies:
Higher TL societies tend to be extremely sensitive to casualities outside of their core worlds. The media - especially media from unfriendly powers and societies - tend to get in the face of the troops a lot. Moreover, there is the problem of arrogance: what happens when people who *shouldn't* have fusion artillery *does* have it? (see: smugglers, high-tech sympathisers with the low-tech people, arms merchants, etc) Or when the Imperial forces start falling into a predictable pattern? (see: the "Black Hawk" incident in Somalia)
Even the war against Iraq is expensive: I have head estimates of as high as a trillion dollars for the current campaign, and unknown billions for the occupation. This is big money, even for the Americans.
For the Imperium,
* Starships are Not Cheap,
* Battledress and Fusion rifles are Not Cheap
* The logistics of maintaining & resupplying High-tech equipment and personnel, parsecs from the nearest High-tech/High-pop world, is Not Cheap
* The numerous media images from the front can cost plenty in "political capital" - especially when Things Go Wrong
* Even pictures of dead enemy soldiers can hurt the Imperium, if the Imperials wish to be loved rather than feared. (It may or may not increase *respect* for the Imperium, if people feel that the Emperor is playing by 'the unspoken rules' every social order has.)
* If the Imperium is a low-birthrate society, then every dead soldier has a real price tag. This is less important if there are only a few low-TL societies in revolt, and more true if many low-TL cultures are anti-Imperial
And all this assumes a High-TL Imperium against a Low TL foe, without fusion power or starfaring technology.
Note that the longer the fight, and the harder the locals, the more likely they will gain access to some high-tech equipment - and the more war-weary your population will be, both of the blood and the money poured down the rat-hole.