Most everyone reading this remembers the aftermath of 9/11/2001. Our country was reeling from the attack and wanted vengeance. As we readied ourselves for sending our military into battle, we also remembered the aftermath of the Viet Nam war. Soldiers came back to a country that saw them as "baby killers" and ignored or reviled them, despite the fact that most of them were drafted, and sent off to fight in a war they didn't start. In 2001, in retrospect, we knew better.
Without the draft, and after years of only minor skirmishes, our military was not as staffed as we would have liked for the initial incursion into Afghanistan, and the later illegal invasion of Iraq. In order to bolster staffing, we offered immigrants a way to fast-track their citizenship if they joined up and fought on behalf of the country they were trying to adopt as their own. That was the promise.