CAIRO—Much of Egypt is under an all-night curfew. An emergency law, the hallmark of a nearly 30-year dictatorship, has resumed, sweeping hundreds of people into detention. Television news channels have rebranded the military's ouster of the nation's first freely elected president as a fight against terrorism.

But while the Arab world's most-populous nation is showing early signs of drifting back toward military-backed autocracy—less than three years after massive protests helped unseat strongman Hosni Mubarak—the moves so far...