These were the stories that put David Drake on the map as an SF writer. He had written (and sold) stories before - his first, "Denkirch", a Lovecraft pastiche, was sold to August Derleth shortly before the latter's death - but after returning from Vietnam, finishing law school, and realizing that he wasn't cut out to be a lawyer, he decided to try and exorcise some of his demons from Vietnam through writing science fiction. The result was "The Butcher's Bill," an unsparing look at Hammer's Slammers, "the toughest regiment that ever killed for a dollar", in action on behalf of a pacifist religious group that really hadn't thought through the consequences of hiring a mercenary armored regiment with a reputation for doing whatever it took to win.
He couldn't sell it. He'd sold a fantasy story to F&SF, and a story about a tank company that found a UFO to Analog, but Ben Bova rejected it on the grounds that he already had Joe Haldeman and Jerry Pournelle writing milSF and didn't think there was room for a third series. Despite this, Drake wrote another Slammers story, "Under The Hammer", about a raw recruit who gets a brutal introduction to war with the Slammers. At this point, the editor of Galaxy, Ejler Jakobsson, got himself fired and was replaced by his assistant, Jim Baen. Baen had recommended that Jakobsson buy the stories but had been overruled; now that Jim was calling the shots, he called Drake's agent and not only bought the stories - he asked for more.
As Drake himself tells it,
"Jim neither understood nor liked the Hammer stories at the time. As he admitted to me later, he bought them simply because they were written with a higher degree of literacy than most of what was submitted to Galaxy, a magazine that paid poorly and late. "Under The Hammer" and "The Butcher's Bill" filled pages which would otherwise have contained stories that would have required heavy editing to bring up to minimum standards of English usage."
Baen bought more stories about the Slammers, and when Baen left for Ace Books in 1977, he asked Drake to do an anthology of the Hammer's Slammers stories. Neither anthologies nor authors' first books sold well in those days, but Hammer's Slammers sold over 90% of its first print run and went through eight more printings before Drake moved the rights to Baen Books, Jim's new publishing house.
Hammer's Slammers (formerly the Frisian Army's Auxiliary Regiment) are a weird blend of the 11th Armored Cavalry Regiment and the French Foreign Legion; yes, the panzers and combat cars are fusion-powered and armed with powerguns, but they're really just expys for the M48 main battle tanks and M113 armored cavalry vehicles that replaced the M114 in the 11th Armored Cavalry. Further parallels to the French Foreign Legion are the Slammers' use of Frisian officers on loan (sometime after tempers had cooled from the events described in "But Loyal To His Own") and the (in this case successful) attempted overthrow of the civilian government by Hammer's men, the aftermath of which is shown in "Coming Home".*
There would be eight more novels in the Slammerverse (two of them, The Voyage and Cross The Stars, not actually involving Hammer's regiment) plus some additional shorter stories; these would all be collected in The Complete Hammer's Slammers, a three-volume set published by Night Shade Books starting in 2006 and republished by Baen starting in 2008.
As with Starship Troopers, Hammer's Slammers inspired a boardgame (from Mayfair games) as well as miniatures & rules for those miniatures. Unlike Starship Troopers, none of Drake's stories have been made into crappy movies by Paul Verhoeven.
*This story contains one of the most chilling lines in the series. When introduced to Colonel Hammer's fiancé, a member of the Old Families that have ruled Friesland since its founding, Hammer's XO Joachim von Steuben introduces himself by saying, "Your family and mine go back a long way, Lady Tromp. Did you know I shot your father on Melpomene? Between the eyes, so he could see it coming."
The blockquote is from Drake's afterword, "Accidentally And By The Back Door" in The Complete Hammer's Slammers, Volume I (Kindle edition)
http://david-drake.com/2010/hammers-slammers-series/