WASHINGTON — President Obama should have declared victory in Afghanistan and begun a withdrawal. His escalation of the war may achieve its goals, but at too great a cost — and without making our nation meaningfully safer from the threat of terrorist attacks.

I hope I’m wrong. But my fundamental question about Obama’s approach was illustrated Thursday by events far from the war zone: In Mogadishu, Somalia, a suicide bomber infiltrated a university graduation ceremony and killed at least 19 people, including three ministers of the Somali government.

I use the term “Somali government” ironically, because there hasn’t really been one since 1991. A long-running, multisided battle for control among heavily armed clans and warlords remains unresolved. The most important recent development in the civil war has been the emergence of a religious-based insurgency, al-Shabab, which now controls a large swath of the country — and which was immediately suspected in Thursday’s bombing.

Where have we seen this movie before?

No, Somalia isn’t a carbon copy of Afghanistan. But it shares the distinction of being a failed state where the ideology of violent, fundamentalist Islam has taken hold and the technique of suicide “martyrdom” attacks is proving effective.

I doubt that Obama’s “extended surge” of 30,000 additional U.S. troops will be successful on its own terms, but let’s assume that it is. According to senior White House officials, this would mean that U.S. and allied forces are able to “degrade” the Taliban to the point where it poses no threat of taking power in Kabul and no longer controls substantial areas of the countryside.

These benchmarks have to be met, the White House says, so that it’s impossible for al-Qaida to return to Afghanistan, establish a base of operations and plan new attacks against the United States and other targets.

My belief is that if the Taliban begins losing ground, many of its fighters will just melt back into the population and bide their time until the president’s July 2011 deadline arrives. At that point, will the Afghan military really be able to stand alone against even a latent Taliban threat? If not, Obama’s deadline will be meaningless and U.S. forces will be stuck in Afghanistan, in large numbers, for the foreseeable future.

But even if the surge works, why wouldn’t al-Qaida — or some like-minded group — simply set up shop in Somalia? Or in Yemen, another failing state? Or in some other wretched corner of the world where central government authority is weak and resentment of the West’s dominant power is high?

Afghanistan happened to be Osama bin Laden’s choice for a headquarters, but he and his top aides were driven out of the country shortly after the U.S. invasion. Al-Qaida is believed to be based in Pakistan now, with the freedom of movement of its leadership severely restricted. The Pakistani government’s obvious reluctance to finish the job is problematic, but I think it’s likely that someday a missile from a Predator drone will find its mark.

The problem is that al-Qaida’s murderous philosophy, which is the real enemy, has no physical base. It can erupt anywhere — even, perhaps, on a heavily guarded U.S. Army post in the middle of Texas.

Look at what’s necessary for the surge in Afghanistan to succeed. President Hamid Karzai has to forswear corruption — which will require more than a stern lecture from Obama. The Afghan military not only has to be trained to fight, but also must expand from its current strength of 92,000 soldiers to as many as 260,000 — a level that Karzai’s weak, cash-strapped government could scarcely afford. And a nation known as the “graveyard of empires” for its legendary resistance to foreign occupation would have to experience a sudden change of heart.

In the end — even if conditions in July 2011 are such that Obama can order a real withdrawal, not a token one — the larger threat of terrorism will remain. The “drain the swamp” approach to fighting terrorism doesn’t work if the virulence can simply infect the next swamp, and the next.

It never made sense to think of the fight against terrorism as a “war” because it’s not possible to defeat a technique or an idea by force of arms. George W. Bush chose a path toward a more or less permanent state of costly, deadly, low-level war. Barack Obama should have taken a different course.

Eugene Robinson is a Washington Post columnist. His e-mail address is eugenerobinson@washpost.com.