WASHINGTON – On his second trip to the Middle East as U.S. commander in chief, President Barack Obama this week will confront a political and strategic landscape nearly unrecognizable from the one he encountered on his first trip to the region shortly after assuming office in 2009.

Gone are the authoritarian regimes and leaders in Tunisia, Egypt, Libya and Yemen, and the once seemingly indestructible Assad regime in Syria is tottering on the brink of collapse.

Uncertainty abounds in the wake of the revolutions that have convulsed the Arab world for the past two years and shaken many of the strong but imperfect pillars of stability on the planet’s most politically volatile patch of land.