Pearl Harbor

American military personnel had intercepted alarming Japanese communications during November and December, 1941. Previously classified "top-secret," those messages are now part of the public record and the National Archives. They reveal an increasingly hostile adversary.

But the warnings actually started ten months before the attack when Ambassador Grew (the U.S. Ambassador to Japan) gave Cordell Hull (the Secretary of State) some troubling news.

TOKYO, January 27, 1941 6 p.m. A member of the Embassy was told by my...colleague that from many quarters, including a Japanese one, he had heard that a surprise attack on Pearl Harbor was planned by the Japanese military forces, in case of "trouble" between Japan and the United States; that the attack would involve the use of all the Japanese military facilities. My colleague said that he was prompted to pass this on because it had come to him from many sources, although the plan seemed fantastic. GREW.

The Office of Naval Intelligence placed "no credence" in these rumors.

Four days before the November 26th meeting in Washington, U.S. forces intercepted an important message from Tokyo to Japan's emissaries in D.C. Secretary Hull was trying to get an agreement between Japan and other countries with interests in Indo-China (like Britain and The Netherlands). The Imperial government was keen to either get an agreement, by November 25, or to proceed alternatively.

Today, of course, we know the alternative course. But the intercepted message is interesting:

...There are reasons beyond your ability to guess why we wanted to settle Japanese-American relations by the 25th, but if within the next three or four days you can finish your conversations with the Americans; if the signing can be completed by the 29th, ...if the pertinent notes can be exchanged; if we can get an understanding with Great Britain and the Netherlands; and in short if everything can be finished, we have decided to wait until that date. This time we mean it, that the deadline absolutely cannot be changed. After that things are automatically going to happen...

Negotiations on the 26th did not go well. "Things" would "automatically" happen. The fleet was en route to Pearl Harbor.