As there was no legitimate declaration of war and as the waging of aggressive war was illegal by 1941, the best remaining argument for the legality of the Pearl Harbor attack is that it was an act of self-defense. This argument comes in two main forms: that the attack was a legitimate anticipatory response to an American ultimatum and that the attack was a legitimate response to the economic pressure and other belligerent action of the United States.
A diplomatic message from Secretary of State Hull ("the Hull Note") of November 26, 1941 harshly stated that the Japanese withdraw all military and police forces from China and from French Indo-China. This was a reversal of the last U.S. proposal submitted by Hull on June 21. Foreign Minister Togo felt that the Hull Note required the denial of the Nanking regime and the abolition of the Tripartite Alliance and thus must at least be "an ultimatum without time limit." However, as the Note did leave Japan an alternative to war, namely concession to American demands, and did not even express an intention to cease negotiations, it cannot be seen as an ultimatum from the perspective of international law. While the Japanese may have felt that they could not accede to such demands and thus were provoked into war, the Hull Note cannot be considered to be a first act of war or even as sufficient provocation to relieve Japan of the responsibility for initiating the use of force in this conflict.
The question that then arises is whether an economic threat can ever rise to such a level that a military attack is an appropriate response. Self-defense entails a right to respond with necessary and proportional force.* It is perhaps arguable that a military attack may be made necessary be a severe economic "attack." While the Kellogg-Briand Pact did not explicitly allow for a right of self-defense, this right was nonetheless assumed. However, it was not defined anywhere in the Pact. Secretary of State Frank B. Kellogg, co-author of the Pact, argued that as "nobody on earth, probably could write an article defining "self defense" or "aggressor" that some country could not get around . . . the only safe thing for any country to do was to judge for itself within its sovereign rights whether it was unjustly attacked and had a right to defend itself . . .." Many have interpreted this statement to mean that aggression is to be determined by the victimized country and thus that economic pressure could rightly be perceived as justifying actions taken in self-defense. However, Kelloggís statements clearly do not necessitate such an understandingóit seems equally plausible that, even if he did intend for nations to interpret the seriousness of attacks for themselves in deciding whether a self-defense response was in order, they were only to consider military attacks in their calculations. However, no decisive answer to this debate exists.
As perceived by the Japanese, the American embargo of oil and other resources necessary to power the Japanese war machine certainly rose to such a level of severity that a military response was justified. "Since 1940 the Americans were visibly trying to deny Japan access to the natural resources of Southeast Asia that had come to be regarded as vital to the empireís security and well-being." These pressures, especially the embargo imposed by President Roosevelt in the summer of 1941, were perceived as a return to the past century of Western pressures and insults beginning with Commodore Perryís "opening" of Japan in 1853-54 and the "unequal treaties" of 1858. Despite these perceptions of impending doom and the slights to the Japanese national pride, military action was not truly necessary in the sense of a lack of other options. The embargo was devised as a means to force Japan to cease its occupation of China; as such, Japan could have ended the economic sanctions by acceding to American demands. Obviously, however, this was not a likely outcome given both the long-standing Japanese desire for control of main-land China and a Greater East Asia Co-Prosperity Sphere "liberated" from the West as well as the militaristic pride that had swept Japan thus far in the twentieth century. While Prime Minister Tojo had "insisted that withdrawal from China ëwould not be in keeping with the dignity of the army,í" the "Japanese dream of empire was perceived sweepingly as a question of survival." Nonetheless, there is a clear distinction between a perception that something is essential to the welfare of the nation-state and a threat truly being sufficiently serious to warrant an act of force in response.
is even less plausible that the attack on Pearl Harbor was proportional
to the economic hardships imposed on Japan by the United States. Although
this analysis is a bit like mixing apples and oranges, that in itself is
illustrative: As military measures are generally considered to be far harsher
than economic measures and a greater violation of the sovereignty of another
state, one must ask whether a military response can ever be proportional
to the witholding of an economic necessity. At least one Japanese scholar
and diplomat, Takeo Iguchi, has voiced this concern, stating that it is
"[h]ard to say from the perspective of international law that exercising
the right of self-defense against economic pressures is considered valid."
While Japan felt that its dreams of further expansion would be brought
to a screeching halt by the American embargo, this "need" cannot be considered
proportional with the destruction suffered by the Pacific Fleet at Pearl
Harbor, intended by Japanese military planners to be as comprehensive as