INGAPORE, Jan. 25 — Shortly after the United States began bombing Afghanistan on Oct. 7, a 30-year- old Indonesian traveling on a false Filipino passport slipped into this tightly controlled city-state carrying a plan to strike back at America. His mission, investigators say, was to activate a "sleeper cell" of Islamic militants who had long been waiting for a call from Al Qaeda's leaders in Afghanistan. This group, which had been loosely organized for eight years, began planning to blow up the embassies of the United States, Israel, Australia and Britain, the investigators say. The plot was foiled when 13 ...