The enemy of my enemy is my friend.

That’s one way to look at the relationship between David Letterman and Conan O’Brien, who reunite tonight on Letterman’s program. The then-unproven O’Brien succeeded Letterman on Late Night in 1993, and later they were head-to-head rivals for seven months during O’Brien’s brief and tumultuous reign as host of The Tonight Show. Both men, at similar points in their television careers, found themselves grasping the short end of the stick in passive-aggressive tangles with Jay Leno and downright hostile relations with NBC over late-night’s showcase program.

But they share more than just battle scars, judging by the warm on-screen exchanges over the years. Back in 1994, when O’Brien was still getting the hang of his new job, Letterman graciously returned to his former digs and blessed his successor at a time when O’Brien was being savaged by critics. “The more I watch the show, I realize that you guys do an incredible amount of comedy…that is very high level,” said Letterman. “I think you’ve really done a great job to carve out a wonderful identity for yourselves. There’s nothing like this show anywhere on television.” Watch that early visit below.

A few months later, Letterman underlined his endorsement by inviting O’Brien to his show.

If not kindred spirits, per se, they share an appreciation for the absurd: Letterman had Stupid Pet Tricks, O’Brien had a masturbating bear; Letterman had Larry “Bud” Melman, O’Brien had Triumph the Insult Comic Dog. In the end, as much as both aspired to be heir to the Tonight Show, both were better suited for an audience other than the one weaned on Carson and Leno.

When O’Brien and Leno locked horns over The Tonight Show in 2010, Letterman didn’t stand idly by. The ugly spat that erupted when NBC decided to reinstate Leno as Tonight Show host resurrected the hard feelings Letterman had buried when he lost out on his dream job in 1992. Almost nightly, he took great joy in poking Leno for his role in the mess.

Technically, the two men remain rivals, with Conan’s 11 p.m. TBS show overlapping with Letterman each night. But they’re hardly bitter rivals, and it’s conceivable that O’Brien — whose cable deal runs through 2015 — might step into Letterman’s shoes once again when his contract with CBS ends in 2014.

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