But as the terror threats subsided, Johnsen said, so too did U.S. funding to support the Yemeni government. And, by 2007, Yemen had once again become a relative safe haven for al Qaeda and its sympathizers.
The "franchised" nature of al Qaeda, in which extremists anywhere and everywhere can pop up and begin using the name for "branding and convenience, has also made it nearly impossible to pin down and eradicate, analyst Cordesman said.
Although the U.S. State Department has called the Yemeni government an important partner in the campaign against terrorism since 9/11, some experts believe non-government actors within Yemen -- sometimes with government approval -- support al Qaeda, according to a Council for Foreign Relations background paper on Yemen.
Al Qaeda's roots in Yemen date to the Soviet-Afghan War from 1979 to 1989. After Saudi Arabia, Yemen was the second largest source of fighters in the Islamist brigade against the Soviets in Afghanistan. Thousands of Yemenis fought and trained at al Qaeda camps in Afghanistan during that time.
Today, al Qaeda in Yemen has proven resilient, despite continuing and recent attempts by the Yemeni government, with help from the United States, to root it out.
"Al Qaeda has done a very good job at building a durable infrastructure that's been able to withstand the loss of key cell leaders," Princeton' Johnsen told ABC News. "So now, al Qaeda in Yemen is so strong and so entrenched that there really is no easy magic missile solution to the problem."
Top U.S. Gen. David Petraeus met unannounced this weekend with Yemeni president Ali Abdallah Saleh, highlighting just how important -- and delicate -- the country has become in the global fight against terrorism.
U.S. Secretary of State Hillary Clinton said Monday that "instability in Yemen is a threat to regional stability and even global stability" and that the United States is committed to strengthening Yemeni anti-terror efforts.
She also supported British Prime Minister Gordon Brown's calls for an international meeting on Yemen to coincide with the upcoming meeting on Afghanistan in London Jan. 28.
ABC News' Kristina Wong contributed to this report.