security clearance
"I’ve been told that Kalitta requires citizenship in order to gain a security clearance for their DoD contracts"
As a green card holder and British national I was treated as a "US person" for security purposes. I worked at a defense contractor's site on a military program without being a citizen and had the same access as a US citizen.
I have no doubt that there are security clearances that would require US citizenship but not all do.