时间上是掐好了的。结果因为日本驻美国大使馆翻译其国内过来的宣战电文速度太慢，给耽误了。 结果日本驻美大使见到美国国务卿，向其送达宣战文书时， 珍珠港已经打起来了。
这种不宣而战的做法， 也为后来美国在太平洋岛上伏击三本56的飞机提供了借口， 因为美国是有法律对敌国领导不搞暗杀的， 但是他们说这一条法律对山本不适合， 因为他先对美国搞不宣而战。 也就是说山本先暗杀了珍珠港几千美国军民。
德国是先对美国宣战。 据说当时美国还担心希特勒会不讲信用（和日本结盟），在珍珠港事件后不先对美国宣战， 这样美国很难去先对德国宣战并且奉行‘对德国作战优先’的政策。 结果希特勒很讲义气， 珍珠港事件后主动对美国宣战了。这样美国顺水推舟马上对德国宣战。
下面摘自维基：
On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, Nazi Germany declared war against the United States, in response to what was claimed to be a series of provocations by the United States government when the US was still officially neutral during World War II. The decision to declare war was made by Adolf Hitler, apparently offhand, almost without consultation. Later that day, the United States declared war on Germany.
