I've heard that Japan had intended to declare war before the bombing of Pearl Harbor, but the declaration was late and so came an hour after the bombing. I have seen it claimed that the late declaration had a significant effect on the conflict afterwards, but I find that hard to believe. The US was already upset with the Japanese invasion of China, so public opinion wasn't that high. Besides, attacking within an hour after a declaration of war seems fast enough that it could have been cast in a negative light anyway. I have also read some sources which claim the December 7th message wasn't a declaration of war at all (source).
Was the lack of a declaration prior to an attack really that important? Would the US (or the international community) had a more favorable response to the attack if the declaration had come just a little bit earlier? Was the timing of the declaration important to the Japanese view of the war?
I can at least point to some documents.
The text of the Japanese 14-point note simply ends
The Japanese Government regrets to have to notify hereby the American Government that in view of the attitude of the American Government it cannot but consider that it is impossible to reach an agreement through further negotiations.
Convention (III) relative to the Opening of Hostilities. The Hague, 18 October 1907, Article 1, is
The contracting Powers recognize that hostilities between themselves must not commence without previous and explicit warning, in the form either of a declaration of war, giving reasons, or of an ultimatum with conditional declaration of war.
I think that saying that negotiations have failed is simply saying that the parties will not agree, in this case neither "previous" nor "explicit".
Roosevelt's address to Congress, paragraphs 2 and 3, were
The United States was at peace with that nation and, at the solicitation of Japan, was still in conversation with its Government and its Emperor looking toward the maintenance of peace in the Pacific. Indeed, 1 hour after Japanese air squadrons had commenced bombing in Oahu, the Japanese Ambassador to tie United States and his colleague delivered to the Secretary of State a formal reply to a recent American message. While this reply stated that it seemed useless to continue the existing diplomatic negotiations, it contained no threat or hint of war or armed attack.
It will be recorded that the distance of Hawaii from Japan makes it obvious that the attack was deliberately planned many days or even weeks ago. During the intervening time the Japanese Government has deliberately sought to deceive the United States by false statements and expressions of hope for continued peace.
Later paragraphs included "a surprise offensive", "the character of the onslaught", "this form of treachery", and "the unprovoked and dastardly attack".
While I have my own conclusions, I have not looked for other sources of the time about whether or how they viewed it as a surprise attack in peacetime, or whether that was important in destroying the previous large isolationist opinion.