Background

"At the beginning of the interstellar contact period, some philosophers and naive scientists thought a time of pacific cooperation between species had come. After all, any society sufficiently evolved to manage space voyage should be advanced enough to put aside barbaric tendencies of its species in respect to combat. - so they stated at least.

...

The most realistic philosophers who predicted the interstellar conflict were as mistaken in their speculation as the naive beings who predicted interstellar peace and the will. Almost without exception, they predicted the battle between spacecrafts were the unique important factor in such wars. The spacecrafts (so they were convinced) could tame any planet where they were orbiting. The ground troops should only be there to control primitive worlds, to act as a security force and to carry out small scale raids.

As we know, this was not what happened. The development of the Xenon screen (also called nuclear shield) changed for ever the face of the war. Without weapons of mass destruction and with cities protected with force fields, to control a planet requires the landing of groundtroops."