Secretary of State Hull, Cordell
The American Secretary of State, Cordell Hull, was aware of the Japanese threat to Pearl Harbor as early as January 1941. On 27 January the ambassador in Tokyo, Joseph C. Grew (Grew, Joseph C.) passed on information that, in the event of war, a massive, surprise attack on Pearl Harbor would be made. This information was passed on to both the Army and the Navy. Negotiations with the Japanese went on throughout the year and Japan sent Kichisaburo Nomura (Nomura, Kichisaburo) to Washington as their ambassador, possibly because he was genuinely eager to find a way to preserve the peace. The continued determination to set up the Greater East Asia Co-Prosperity Sphere led to the invasion of French Indo-China and to an oil embargo which, on 26 November, Hull made proposals to bring to an end. As far as reading diplomatic codes went, American espionage was effective and Hull was aware of the content of instructions sent to Nomura on 6 December before the Ambassador himself had the message in clear form. He was thus aware that the timing of an appointment made for a meeting with Nomura, 1300 Eastern Standard Time, 0730 in Hawaii, was significant, as were the MAGIC decoders. The warning sent to Lieutenant-General Walter C. Short (Short, Walter C.) was delivered by RCA Messenger Tadao Fuchikami (Fuchikami, Tadao) too late to be of use. The meeting with Hull was delayed to 1345 EST at Nomura's request because the decoding of the message from Tokyo was incomplete. Nomura and Saburo Kurusa arrived at the State Department at 1405 and were shown to a diplomatic waiting room. At that point President Franklin D. Roosevelt telephoned Hull with the news that the attack on Pearl Harbor had begun. At 1420 the Japanese entered Hull's office and handed him their message, which Hull had already seen, and the American made a show of reading it. When Nomura tried to speak, Hull stopped him with an upheld hand, told him that he, Hull, had always spoken the truth and that the Japanese had responded with lies and waved his visitors from the room.
You can find out more about Cordell Hull in the following Osprey book: