For a time, early in World War II, Admiral Darlin was the most important person in the world. As commander of the French fleet. he perhaps held the balance of power. What would he do with the fleet after the Nazis marched into Paris in spring 1940? Would he present it to Hitler? Would he turn it over to the Allies? Or would he scuttle it?     He ended up collaborating with both sides, a dangerous trick. When the Allies decided they needed him, FDR was accused of dealing with the Devil.     But who gave the order for his assassination?