Kristin Cavallari -- Hard Corps Date to Marine Ball

Kristin Cavallari served her country with pride last night ... looking smokin' hot for her date at the Marine Corps Ball in Temecula, CA.

The MTV star kept her promise to Lance Cpl. Jonathan Burkett ... after she accepted his Twitter invite a while back ... and she didn't disappoint.

We're told Kristin was a helluva date -- taking pictures with anyone who asked. KC didn't try to pull any of that Hollywood early exit crap -- we're told she stayed at the party until they shut the place down.

Burkett tells us, "[Kristin] was awesome for sure. She looked very beautiful. [Jay] Cutler is a lucky guy."