The war didn't start on September 11. The simultaneous bombing of our embassies in Kenya and Tanzania, on August 7, 1998, was just the latest attack in a war al Qaeda had been waging against us since the early 1990s. But we didn't treat it as a war until hijacked jets slammed into the World Trade Centers and the Pentagon. Steve Coll details the many policy failures that led to September 11 in " Ghost Wars." It's a book I highly recommend.