In 1907 at the Hague Convention III, it was agreed that a Declaration of War must be submitted and received prior to hostilities. In this situation, the Japanese did submit a document, but it was at the last possible moment and there were some issues with the document, and it getting delivered on time. The telegram arrived in time, but was in 14 parts, and there were issues in getting it properly translated which delayed the message until after the attack.
The document was originally to be delivered to the U.S. Embassy in Tokyo, but a last minute decision made by the Japanese government was to deliver it to Secretary Hull in Washington.
The telegram was finally decoded and stated that the Japanese could not further negotiate with the U.S., but did not mention the outbreak of hostilities.
The Japanese government knew the telegram was not a true Declaration of War, yet this was the only message sent prior to the attack.
A true Declaration of War was drafted, but never sent. This was discovered in 1997. Reasonable implication is the Japanese did not want to warn the U.S., but felt honor-bound to send something.
Japan did not want the U.S. naval fleet to interfere when it attacked the British and Dutch Indies. The Japanese needed these lands for the oil since the U.S. had put an embargo on oil shipments to Japan. The best harbor in the this area, Manila Philippines, had to be seized so the U.S. did not cut off the oil. Their plan was to deal the U.S. a heavy blow, give themselves a year or two to expand, and have the U.S. and Great Britain admit losses of these lands. They did not expect the U.S. industrial machine to react so quickly.
Hope this helps.