...The UFP would not lose any face if it turned out that its forces were preemptively slaughtering the enemy. That counts as gaining face, for all of the Star Trek cultures we know of. Well, except for the Halkans. And it doesn't sound likely that the UFP would worry much about the Halkans now that they can recrystallize their dilithium...
In this particular neighborhood, the UFP would not want to send the message "we are sorry we killed people". It would want to send the message "we killed people, and we're damn good at it". The bellicose players in the region would tremble, whereas those fed up with bellicose players would not particularly mind finding out the Feds were badasses - they'd separately evaluate the merits of being on the good side of the UFP, and then find delight in that the folks who have the least reprehensible policies are also the ones with the greatest military prowess and audacity.
Timo Saloniemi
Except we didn't end up at war with Cardassia until about a decade later. So yes, we would have been apologizing for killing Cardassians in Cardassian space in a time where we were not at war. That is a no no.