[southland]

I believe FDR did know.
We had broke Jap code, I believe.

[Eric Pode of Croydon]

We had broken their diplomatic code, but it took a long time to decrypt the final "this means war" message and the man who had it (LCdr Kramer) did not have the clout to immediately get through to the very few top brass and politicians who were authorized to read the intercepts.

And we actually decrypted the message faster than the Japanese embassy did, which is why they presented their declaration of war to Cordell Hull after the attack on Pearl rather than just a few minutes before as had been the plan.

The best description of this IMO is in the first chapter of The Codebreakers by David Kahn.