U.S. officials specifically discounted rumors that were circulating late Sunday that Sissi is an Islamist with secret connections to the Muslim Brotherhood. To the contrary, officials say, Sissi is well known to the U.S. military after spending a year of professional training in the United States and was regarded as a generally effective head of military intelligence…

The U.S. view is that the replacement of aging top military leaders, in itself, isn’t worrying. But they would be concerned if Morsi moved to make changes in Egypt’s judiciary, which has been an important independent center of power since the Tahrir Square revolution that deposed Mubarak in February 2011. Worries about the judiciary were prompted by another Morsi move Sunday — to appoint senior judge Mahmoud Mekki as vice president. The fear is that Mekki, as a former jurist, might reject rulings by the courts…

The Israelis are said to be more concerned by Sunday’s purge, worrying that Morsi is taking a series of steps that may be leading toward a collision with Jerusalem. But for both the United States and Israel, watching developments in Egypt is a bit like riding a tiger — potentially very dangerous, but impossible to steer.