[_delirium]

I don't believe there's an explicit quota (I could be wrong), but the Vietnam Era Veterans' Readjustment Assistance Act of 1974 added certain affirmative-action requirements relating to government contractors hiring Vietnam vets, which was later expanded to all vets.

From a brief skim of since-1974 stuff on the subject, it looks like much of it may result from bipartisan-coalition politics: you often see "disabled and veterans" joined together in laws and rulemaking as a group, probably as horsetrading where liberals get something pro-disabled, and conservatives get something pro-veteran.