Conan O’Brien and David Letterman Mock Leno on ‘Late Show’ (Video)

"You know Jay's watching right now," O'Brien cracks of his late-night nemesis

Nothing like seeing a couple of old pals get together again after a prolonged absence … and savage a common enemy.

Conan O'Brien made his first visit to "Late Show With David Letterman" since 1999, and naturally the conversation turned  to Leno, to whom both have lost out on the "Tonight Show" gig to Leno.

"You know Jay's watching right now. He's getting a live feed in a satellite truck right now. He is watching every second," Conan cracked, before launching into a wicked Leno impression.

"Dave looks unhappy," O'Brien mocked, in Leno's trademark nasally tone. "He's gonna pack it in soon."

Letterman went on to jokingly note how much he enjoyed the 2010 "Tonight Show" snafu, during which Conan was replaced by Leno after a brief run on the late-night staple.

"I was delighted by everything that happened — except for you losing your job," Letterman offered.

""The only consolation I took during that period was that you were happy," O'Brien shot back.

O'Brien's "Late Show" appearance airs Thursday night. Until then, watch the latest salvo in the late-night wars unfold in the preview clip.