The first time I remember hearing it was when (j)urinalists complained about one of their ilk being killed in the battle of Bhagdad. US troops were taking fire from a hotel so a tank blew a big hole in it and a (j)urinalist was killed...,...(no loss imo)
IIRC, there was some kind of court case brought about over this but I don't remember the outcome.
Proportional response, I seem to recall, was first used in the nuke context, as in instead of massive retaliation we would respond in a proportional manner.
“Where the heck did the concept of “proportional response” come from?”
Probably from weaker, but more clever enemies - both external and internal.
Experience has shown that Russia’s brutal, grossly disproportionate, and completely indiscriminate responses to threats has worked very well for deterring future attacks. I could make a case that this policy SAVES lives.