In the acknowledgments to his new book, David Rieff jokes that the phrase he is least likely to utter is “I don’t have an opinion about that.” But “At the Point of a Gun” (Simon and Schuster, 270 pages, $24), a collection of his essays and articles about recent humanitarian disasters and the West’s responses to them, illustrates the danger of having too many opinions: Eventually they cancel each other out, and all you are left with is a mood. Mr. Rieff starts out indignant at the failure of the “international community,” led by the United States, to intervene in the Rwanda genocide; he ends up indignant at the decision of the United States to intervene in Iraq.

Along the way, he is first sanguine, then pessimistic about the NATO war to stop the ethnic cleansing in Kosovo; he is alternately hopeful and despairing about the potential of the United Nations to do good in the world. Mr. Rieff’s predictions and prescriptions change from one article to the next, and are challenged in turn by the brief postscripts he has added for their publication in book form. The only thing that remains constant is Mr. Rieff’s justified grief about the fallen state of the world, and his insistence that, in the book’s final, emblematic words, “the future seems very bleak indeed … and growing bleaker by the day.”

For Mr. Rieff, an American with a global conscience and perspective – informed by his years as a foreign correspondent in some of the world’s worst disaster zones, including postwar Baghdad – the tormenting problem is whether to view the United States as a force for good or for evil. Over the course of the last decade, Mr. Rieff explains in his introduction, he has moved steadily away from his former hopefulness about what he called, in a 1999 essay, “a new age of liberal imperialism.”

Back then, mindful of the West’s failure to intervene in Bosnia and Rwanda until it was too late, Mr. Rieff proposed that we bring back the old post-World War I mandatory system, in which Western powers would take custody over lawless regions. “Is this proposal tantamount to calling for a recolonization of part of the world? … Clearly it is,” he acknowledged; but “however controversial it may be to say this, our choice at the millennium seems to boil down to imperialism or barbarism.” Yet now, just five years later, Mr. Rieff seems to reject not just a new imperialism, but any use of American military power. “This book,” he writes, “is largely an argument against … my previous conviction that humanitarian military intervention, whether to alleviate massive suffering or rectify grave human rights violations, should be the norm.”

So, at least, Mr. Rieff claims in his introduction. Yet when we come to the actual articles he has collected, it is hard to tell exactly where Mr. Rieff’s opinions have changed, or why. He is still pessimistic about the power of the U.N. to solve the world’s problems, and still endorses his apocalyptic forecast for sub-Saharan Africa. His 1996 essay “An Age of Genocide” is still a convincing indictment of the Western policies that allowed the Rwanda horror to take place. He does not even seem to wholeheartedly renounce his support, in a 1999 article on “Lost Kosovo,” for the NATO intervention there.

Why, then, does Mr. Rieff insist on a change of heart that “At the Point of a Gun” does little to explain? The answer comes in the book’s last section, which deals with the American invasion of Iraq. Mr. Rieff’s reported pieces on the failure of America’s postwar planning, the human cost of sanctions in the 1990s, and the new militancy of the country’s Shiites – all of which appeared in the New York Times Magazine over the last two years – add up to a comprehensive denial that the United States can use its power either wisely or well.

“The reality of Iraq as I witnessed it from my worm’s eye view on the ground,” Mr. Rieff says, “always seemed worse than anything I was able to write about it.” No wonder that, “after the experience of postwar Iraq,” Mr. Rieff was left feeling that “there are very few just wars … what I witnessed on the ground in Iraq was the speed with which altruism can become barbarism.”

The problem with “At the Point of a Gun,” however, is that Mr. Rieff’s current view of the Iraq war colors his views of Kosovo, Bosnia, and Rwanda, in a way that is not fully defended or perhaps defensible. For Mr. Rieff writes about Iraq as though it, too, were a humanitarian war, an intervention to protect the human rights of Saddam’s victims and punish their tormentor. But while this was one result of the war – perhaps its only unambiguously praiseworthy result – it was not the reason why the United States invaded Iraq. The reasons had to do with national security and geopolitical strategy, and centered on the assumption that Saddam was close to acquiring nuclear weapons.

The discovery that this was not the case has retrospectively undermined the argument for war, and the chaos of the postwar occupation may yet undermine the strategic benefit we might have gained from the change of regime. But these are political, not humanitarian, questions, and Mr. Rieff does not draw any logical connection between our action in Iraq and our failure to act in Rwanda. It is disturbing to see him, in his jaded postwar mood, more or less dismiss the ongoing crisis in Darfur (“with every passing day, the estimates of how many people might die in Darfur seemed to increase”), on the principle of once bitten, twice shy. The worst possible outcome of the Iraq war would be if it convinced all Americans, as it has convinced Mr. Rieff, that any use of American power for good should be written off as utopian adventurism.