When al Qaeda was just a startup terrorist enterprise, the Sudanese government offered the group safe harbor. But after a few years, Saudi Arabia, Egypt and the United States began to take notice of what was going on in that little incubator.

They turned up the heat.

By 1996, Osama bin Laden was looking to relocate. No longer feeling safe in Sudan, he moved al Qaeda headquarters to Jalalabad, Afghanistan.

One of the greatest assets any would-be transnational terrorist group can acquire is a secure base of operations. Of the dozens of terrorist attacks aimed the United States since Sept. 11, 2001, about half have involved individuals who "graduated" from a terrorist training camp or sanctuary.

After Sept. 11, the U.S. and its allies hounded al Qaeda out of Afghanistan. Later, with key operatives dodging drone strikes in Pakistan, bin Laden elected to establish an alternative base of operations in case his gang needed to abandon South Asia. His choice of location: Yemen.

For bin Laden, there was a lot to like about Yemen. For starters, the country had become a holding pen for al Qaeda operatives chased out of Saudi Arabia, so a cadre of devotees was already on site.

Also, there was lots of ungoverned space available. With a Houthi rebellion in the north, a secessionist movement in the south, and a weak and venal government in the middle, Yemen presented just the kind of distracted, chaotic neighborhood in which a terrorist organization could settle down and, ultimately, prosper.

Best of all, terrorist homebuyers, Yemen offers "location, location, location." It sits at the crossroads of the Middle East and North Africa. And just across the Gulf of Aden lies Somalia, another failed state with a very friendly neighbor: the prominent terrorist group al-Shabaab.

Al Qaeda in the Arabian Peninsula is now well established in Yemen tribal areas. One of its cells, known as the Foreign Operations Unit, focuses its efforts on conducting al Qaeda's business abroad.

At least three attacks on the United States can be linked to the Foreign Operations Unit. Maj. Nidal Malik Hasan of Fort Hood infamy colluded with Anwar al-Awlaki, a U.S.-born cleric now thought to be running the unit. The Christmas underwear bomber and the "ink cartridge bomb plot" also have links to Yemen.

The U.S. government has issued a "capture-kill" order for al-Awlaki. He is a dangerous dude. And he keeps bad company. His colleagues in the unit include Ibrahim Hassan Asiri, whose talents and creativity as a bomb-maker are near legendary.

A second American-born member of the unit is Samir Khan. Khan is believed play a pivotal role in the production of English-language extremist magazine Inspire. While al-Awlaki plans terrorist attacks, Khan toils tirelessly on the Internet, trying to inspire "lone wolves" to take up arms against "the great Satan."

Of course, the Foreign Operations Unit can do little without foot soldiers. And there seems to be no shortage of those. As the Yemeni government was melting under the heat of the Arab Spring, 63 al Qaeda operatives staged a breakout from a Yemeni prison in Hadramout. They all remain at large -- and doubtless in touch with the unit.

What if the administration quits Afghanistan before the job is done? Relax the pressure there and al Qaeda could reconstitute itself in South Asia in short order. But it certainly wouldn't close up shop in Yemen. Rather, the group would then have two solid support bases to work from.

The White House answer for dealing that eventuality: lobbing drone strikes from afar. That's even less promising than the air-power "solution" to the Moammar Gadhafi problem in Libya.

James Jay Carafano is a senior research fellow for national security at the Heritage Foundation.