Introducing such beasts as space carriers (mentioned in Emperor of the Fading Suns) would finally give the houses the ability to transport troops as seen in the Dune series where house Atreides moves its entire army from Caladan to Arrakis.
There's a very good discussion regarding planetarry assaults here.
Introducing such beasts as space carriers (mentioned in Emperor of the Fading Suns) would finally give the houses the ability to transport troops as seen in the Dune series where house Atreides moves its entire army from Caladan to Arrakis.
But remaining with the present numbers and within canon possibilities there is a logical explanation as to how a limited number of technologically advanced troops could infact take over a planet with a population of 100 million or more.
We have to remember that the Known Worlds are mostly populated by a people that live in medieval conditions and have a similar frameset. These folks, as our Earth medieval counterparts, have no concept of belonging to a nation/realm/planet etc and as in our middle ages they usually don't care less who controls their lives. After all, giving up half your income/products to one noble or another doesn't make a difference in the end. Its semantics.
So no matter what the canon says (that all factions except the Hazat and the League) use the serfs as cannon fodder, I would disagree, for such troops would no only be totally inefficient (send a hundred thousand against a power armor legion and they'll all be cut to shreds) but arming the peasants and inticing them to do battle is potentially detrimental to the social order in general. Remember that we are speaking of a neofeudal society here. While it was common for soldiers to be used as cannon fodder in World War I (in France) or World War II (in Russia) these people were partly inspired (or it was at least justified) by a "national feeling". Serfs on the other hand have absolutely nothing to fight for, except when they are rebelling against unjust taxes (in their eyes of course) and I can find no example in our history that a noble faction would use serfs in a military engagement. (unless the enemy were the Turks in Central and Southeastern Europe)
It is the medieval division into
oratores - those that pray
bellatores - those that fight
laboratores - those that work
that makes the system work and allowing the "workers" to fight is like sending out a signal that you're obsolete. That's what happened in the 18th century and as a result we had athe French revolution that caused a collapse of the feudal system.
Thus the first "noble" law in this post republican era would be to completely shut out the serfs from waging war. Once they realize that they "can" fight they might start thinking that maybe the "nobles" whose duty it is to protect them from enemies aren't that necessary. What then?
Now if we look at war as a clearly "noble enterprise" we can explain that a world of some 100 000 would have about (in analogy to medieval Britain which could field 5000 knights from a population of 1.4 million - from Doomsday book by William the Conqueror 1085AD) 150 000 knights spread out and some 50 000 might be available to respond to an invasion. Since most of them won't be equipped with top level tech it is reasonable to assume that 11 000 power armored commandos (or similar) could square off against the planetary defenders on equal terms especially if they also have air superiority.
That a high tech army can take out a more numerous army and control a siezable population we can see from the recent war in Iraq. The coalition had some 250 000 troops and the Iraqi army was I believe some 100 000 stronger. And in the end the coalition achieved an easy victory and took control of a country with 24 million people (which have access to TV, AK-47, electricity etc as opposed to our serfs with plows)
By controling the vital infrastructure and command centers you can control the entire world or parts of it. Add to it that many factions exist on every planet, no matter whose house it belongs to you can always find others willing to "join" and share the spoils.