US President Barack Obama was poised on Friday to welcome Egyptian President Hosni Mubarak's decision to step down but Washington now faces deep uncertainty and big challenges in dealing with Egypt's potentially volatile transition of power.
Obama, who had struggled to calibrate his response to a popular uprising against Mubarak's long authoritarian rule, was due to deliver a statement after Mubarak handed power over to the army.
Washington had walked a fine line since the demonstrations began, endorsing the democratic aspirations of the protesters but trying not to openly abandon a long-time ally or encourage an abrupt upheaval.
Obama now faces the challenge of backing broad political reform in the Arab world's most populous country while keeping Islamists from ending up with enough power to undermine US interests in the region.
He will face the test of keeping the power shift in Cairo from unnerving Middle East allies like Saudi Arabia and Israel or emboldening foes like Iran and al Qaeda.