Robert Kagan published "Power and Weakness" in the June 2002 issue of Policy Review. The article became very influential, and justly so, for its controversial claims about diverging American and European views of power in the realm of foreign policy. A year later Kagan expanded the article into a book entitled Of Paradise and Power: America and Europe in the New World Order.
The paperback edition, which recently appeared in bookstores, has been supplemented by a 54-page afterword. You can download a PDF of the afterword (260 KB) if you already have the hardback edition and don't want to purchase the paperback.
Kagan adapted the new afterword for an article in the March/April 2004 issue of Foreign Affairs. Only a preview is available on that site, but the entire article was re-printed by The New York Times.