Eric Alterman has this to say about George Kennan and John Gaddis:

Had Kennan not lived so long, Gaddis might have done a fair job as his biographer. But as Kennan, despite remaining an old-fashioned conservative in the tradition of Walter Lippmann and Hans Morgenthau, moved further and further to the dovish/diplomatic wing of foreign policy debate, his biographer rushed headlong in the opposite direction. Kennan, for instance, strongly opposed Bush’s Iraq adventure, while Gaddis sounded like Dick Cheney on steroids during this period. Cautioning Democrats not to take issue with intellectual currents underlying Bush’s foreign policy, Gaddis argued: “The world now must be made safe for democracy, and this is no longer just an idealistic issue; it’s an issue of our own safety,” later adding, “A global commitment to remove remaining tyrants could complete a process Americans began 232 years ago.”

The result, sadly, is a biography, George F. Kennan: An American Life, in which the author not only sides emotionally and intellectually with his subject’s adversaries but, in many instances, does not even try to do justice to his subject’s arguments.

It must be significant that Kennan agreed to Gaddis as his biographer before Gaddis wrote The Long Peace – before that, I suppose it was not clear how different were their respective directions.