Anwar al-Awlaki could have had a trial anytime he wanted. He simply had to go to the American Embassy and turn himself in. As things stood when he was killed, it was impossible to bring him back for a trial without an unacceptably high risks to the people in the armed forces who would have had to capture him. If you wanted some insight on the complexities of operating within Yemen, I suggest The Black Banners by Ali Soufan. The subject of the book is the events leading up to 9/11 and Soufan was one of the FBI's top agents and strong critic enhanced interrogation. He invested the USS Cole bombing in Yemen and he covers the difficulty of working in country quite well.