NO ONE saw it coming. Three month ago the rulers of Tunisia and Egypt, Libya and Bahrain seemed firmly in control. Dissent of any kind, let alone revolutionary change, was nowhere on the horizon.Now it's anybody's guess which country will be next. This is not unusual. The US military tries to predict political instability, and the result, while secret, have apparently been poor. "We have never once gotten it right," Robert Gates,US Secretary of Defense, said last week