Was the Attack on Pearl Harbor Illegal?
Causing the deaths of 2,403 individuals would in most cases be considered illegal without question. In times of war, however, the rules can be a bit murkier. In the Pacific Theater, over 100,000 Americans died in battle, but these deaths weren’t war crimes. They were the tragic but expected product of a conflict that was recognized by all parties involved as a legitimate campaign. But the act that started the War in the Pacific probably did not fall within the definition of a legal act of war. So, was the attack on Pearl Harbor illegal?
On December 7, 1941, without warning, the Imperial Japanese Navy launched an attack on the United States, striking the Pearl Harbor naval base in the Territory of Hawaii. The unprovoked attack left 2,403 Americans dead and resulted in a declaration of war against Japan the following day. The question of the legality of the attack on Pearl Harbor wasn’t foremost in most people's minds at the time it unfolded, but after the war, as Japanese officials were being brought up on war crimes charges before the International Military Tribunal for the Far East, it became central to many of the trials. For the answer to that question, we need to look at the international agreements in place at the time of the attack.
What Was the Law in 1941?
The war crimes prosecutors looked to Article 1 of the 1907 Hague Convention III - The Opening of Hostilities. According to the article, an attack performed “without previous and explicit warning” was against international law. Though Japan did compose a warning to the United States, it was not delivered until an hour after the attack was launched. Additionally, Article 2 of the 1907 Hague Convention states “[t]he existence of a state of war must be notified to the neutral powers without delay, and shall not take effect in regard to them until after the receipt of a notification.”
Since Japan and the United States had been engaged in negotiations over peace in the Pacific, the apparent breakdown of the negotiations couldn't be considered enough of a warning for the December 7, 1941 attack to be legal under the 1907 Hague Convention.
The Kellog-Briand Pact of 1928 also deems the attack on Pearl Harbor as an illegal act. According to the pact, which was signed by both the United States and Japan, an act of war would not be used to resolve “disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them.” Japan attacked the United States in response to the embargoes placed on vital imports, which the Japanese likely saw as an act of aggression. The mastermind behind the attack, Isoroku Yamamoto, went into it knowing that, if Japan lost, he would be tried as a war criminal.
Ultimately, Prime Minister Hideki Tojo, Foreign Minister Shigenori Togo, Minister of the Navy Shigetaro Shimada, and Chief of Naval General Staff Osami Nagano were all charged with crimes against peace and murder in connection with the attack on Pearl Harbor. The Tokyo War Crimes Trials unequivocally declared the attack on Pearl Harbor illegal.