Finally got myself a copy of vol 1 of Hammer's Slammers, and I can definitely see the influence. (A) putting mercenaries at the center of the action, regulated with a bonding agency, and (B) having monocultural colonies that each represent a modern day country.
Side note: the constant slaughtering of civilians in the Slammers series is starting to get to me... It's almost every story: let's massacre the surrendering rebels, let's drop a nuke on the villagers, let's shoot their families for not surrendering fast enough. I think it need to out this down and read something else...
David Drake was in
The Blackhorse in Vietnam, during a time when bodycounts, freefire zones, and other such genocidal tactics were being used by the US government to fight the vietcong. The Slammers basically take said tactics to the extreme, mirroring the attitudes many soldiers had towards them. the combination of the vietcong insurgency being able to blend into the civilian population (and usually getting lots of support, due to the way the US forces alienated the populace with their tactics), combined with the actions they were asked to do, caused many troops to dehumanize both the enemy and the native vietnamese civilians in their perceptions.
I don't know if Drake participated in such tactics (Armored units didn't get used for that much) but he certainly would have seen and heard about them, met soldiers like that. given he's said in some of the forwards for his novels that writing the Slammers started as a sort of self-therapy, i suspect that the experiences effected him deeply.
or the TL:DR version.. the Slammers novels are the kind of Vietnam Expy that
Ideal War had wanted to be but failed to become. and are meant to show that war is hell on all sides.