Peter Beinart argues that a successful war in Iraq can help the war against terrorism rather than hurting it, as has been argued by Al Gore, among others.

State Department officials say no country has even privately threatened to cut off anti-terrorism cooperation over an Iraq war. In fact, the German government, fearful that its vocal antiwar stance makes it look like an unreliable ally, has actually increased its antiterrorism assistance--allowing an Al Qaeda suspect to be extradited from Pakistan to the U.S. even though Germany has legal jurisdiction and promising to expand its role in Afghan peacekeeping.
Germany is acting rationally. Few governments want to incur Washington's wrath, and those that oppose America's war against Saddam are unlikely to compound the diplomatic damage by simultaneously stiffing us on the war on terrorism. That's especially true because shared intelligence flows both ways, and governments in places like Russia, Egypt, and Pakistan are at least as threatened by Islamist terrorism as the United States. Some dovish commentators worry that even if those governments want to maintain cooperation, public opinion will force them to cut it off. But intelligence cooperation is almost by definition covert; virtually no government policy is less subject to public opinion. If Hosni Mubarak really feels pressure to throw Egypt's anti-American masses a bone in the wake of a U.S. invasion of Iraq, it's more likely he'll announce a boycott of U.S. products or publicly spurn a meeting with President Bush than stop his regime's clandestine cooperation with CIA personnel tracking Al Qaeda fanatics in Egypt.
The third way a war in Iraq could undermine the war on terrorism, according to Kennedy, is by "swell[ing] the ranks of Al Qaeda sympathizers and trigger[ing] an escalation in terrorist acts." But while Al Qaeda might be stronger during a war with Iraq, it would probably be weaker after one. Take the war in Afghanistan as a model. U.S. bombing sparked anti-American protests in much of the Muslim world. But once the U.S. toppled the Taliban, the protests diminished dramatically. For one thing, would-be Al Qaeda recruits saw the hopelessness of confronting American power. For another, they saw that the people of Kabul weren't on their side.
An American victory in Iraq would probably have a similar effect. Once we win--which pretty much everyone concedes we will--the anti-American protests will end. The image of the United States as a paper tiger, which animated Islamists in the 1990s, will be dealt another blow. And the image of the United States suffocating the Iraqi people through sanctions, long a staple of Al Qaeda propaganda, will likely be replaced by images of American GIs being welcomed as liberators. It's true that over time the euphoria might dissipate, and an American peacekeeping force in Iraq could generate Arab resentment. But with Saddam out of power, the United States might be able to withdraw its troops from another part of the Middle East: Saudi Arabia. And given that it is the presence of U.S. troops near Mecca and Medina that led bin Laden to turn against the United States in the first place, an American withdrawal from Saudi Arabia would probably do more to undermine Islamist recruiting than an American occupation of Iraq would do to fuel it.

I think Beinart is right. More generally, I think that the assumption that a war with Iraq will hurt the war on terrorism is usually exactly that - an assumption, with little evidence cited. As Jonah Goldberg points out:

Taking America's side in a war is a very public act; cooperating with America's law and intelligence services is a very private affair. The ability to publicly snub America on Iraq while privately earning America's gratitude in the war on terror may seem like a boon to many world leaders. Pakistan's Musharaf would probably leap at the opportunity to denounce a war on a Muslim country — with a wink and a nod from the U.S. — while quietly rounding up members of al Qaeda and currying favor with America. Indeed, this is pretty much what Yemen, Saudi Arabia, Egypt, Syria, Indonesia, Russia, and France have been doing for most of the last year — denouncing American belligerence toward Iraq while cooperating fully with the U.S. in the fight against al Qaeda.
Sure, if the U.S. went to war with Iraq, some nations might stop cooperating in the fight against al Qaeda. But you can't simply assert that this is so. Because the counter-argument is at least as compelling.
As for the second leg of the argument, I just don't get it. The war on terrorism/al Qaeda is not an intensively military war, at least outside Afghanistan. The numbers of military troops dedicated to the fight against al Qaeda inside Afghanistan is between four and five thousand. Roughly the same number of troops are spread out throughout the rest of the region, as well as in places like Yemen. The current military was built up on the assumption that the United States might have to wage and win two full-blown wars simultaneously, i.e., fight North Korea and Iraq at the same time. Now that the Taliban has been deposed, the war on terrorism doesn't use many tanks, aircraft carriers, artillery batteries, etc. The idea that a war against Iraq would drain the war on terrorism is simply not true if you're talking about materiel and troops.
Now, it is likely that a war on Iraq would divert some special forces and intelligence assets from Afghanistan to the Persian Gulf. Fair enough. But do we really want to make the argument that we cannot go to war because a few hundred men are stretched thin? We have an active-duty military of about 1.4 million people, and you're telling me they might as well stay in the barracks if a subgroup smaller than a softball league is busy? And if it's a matter of too few spy drones and cruise missiles, the answer is pretty simple: Buy more.