Needless to say, Joe Lieberman has every right to back John McCain for president, and doesn't have to justify himself to me. However, he is justifying himself, which makes his reasons fair game. Let the fun begin.
As nonsensical concepts go, a bipartisan or nonpartisan presidential endorsement is a doozy. Anyone running for president with a serious chance will have a party, or similar organization, behind him or her. McCain has the Republican Party. JIL isn't choosing country over party, as he states. He's siding with an opposing party over the one he's belonged to for most of his life, much as Lie Like Hell Zell did back in '04. Now, this isn't the profoundest level of betrayal there is. It's also not some exalted, noble impulse.
As to his reasoning, Lieberman is one of those politicians who thinks that because we have so many hammers, every problem in the world is a nail. Like many life-long civilians, he has a rather romanticized view of war, and seems baffled by anyone in public life who would say no to it. A lot of veterans, even if they still believe war is necessary, would disabuse you of this notion. John McCain is not one of those, which is why he and Lieberman get along so well.
As to the Clintonistas that Lieberman is trying to woo to McCain's corner, I hope they remember that he was doing this whole "more in sadness than in anger" schtick back in March, back when the primaries were still raging. Meaning that if Hillary had clinched the nomination, Lieberman would be making his pitch to disaffected Obamaphiles.
Hey, you never know until you try.