It’s been 20 years since a U.S. Secretary of State traveled to Yemen, and Hillary Clinton’s appearance today in the capital city of Sana’a vividly underscores the Obama administration’s fast-growing concern, as detailed by John Brennan last month, about the threat Yemeni-based al Qaeda operatives pose to America. (The Christmas Day 2009 “underwear bomber” originated from Yemen, as did the unexploded cargo bombs of late October.) Clinton’s visit comes a few days after a series of al Qaeda attacks killed more than 20 Yemeni soldiers, demonstrating how tough the terrorist group will be to root out. Yemen isn’t likely to become another Afghanistan for the U.S.: It lacks the strategic significance of Afghanistan, with its proximity and ties to nuclear-armed Pakistan. But it is emerging as something of a test case in whether and how we can prevent al Qaeda from choosing a country with a weak central government and setting up a base there from which to strike Europe and America.