Prior to the attack on Pearl Harbor there was no blockade of Japan. Five months before the Pearl Harbor attack the Dutch, English, and Americans announced an embargo on the sale of oil, steel and scrap metal. An embargo is not an act of war; it is an economic and political tool, enforced primarily through laws, regulations, and diplomatic pressure. Its intent is always to solicit acceptable behavior, which in July 1941 meant for Japan to cease its war actions in Manchuria, China, and Indonesia.  All this before Pearl Harbor.

A naval blockade is an act of war. Naval blockades come in two forms: the embargo of selected items and a complete shutdown of ships with cargo of any type. World War I was enmeshed in the endless slugfest of trench warfare when Britain instituted a complete naval blockade to cut off Germany’s access to food, raw materials, and supplies. The Royal Navy blockaded the North Sea, intercepting ships heading to Germany and occupied countries in Europe. This led to severe food shortages and malnutrition among German civilians. It is estimated that 400,000–750,000 German civilian deaths were due to starvation and disease. The blockade contributed significantly to domestic unrest and the eventual German collapse in 1918. The blockade continued even after the armistice (Nov 11, 1918) until the signing of the Treaty of Versailles (June 28, 1919).

Germany’s unrestricted submarine warfare in WWII was an attempt to blockade England from receiving needed supplies including food. Before the war, 70% of Britain’s food supply was imported. In 1942, as German U-boat attacks during the Battle of the Atlantic reached their peak, imports of meat, sugar, fruit, and cereals were drastically cut. This led to severe rationing. Britain successfully avoided famine through a mix of rationing, domestic production, and American aid which began to increase significantly by late 1942 as the Battle of the Atlantic began to turn in favor of the allies.

Submarines were the key and essential element of a blockade of the Japanese home islands. Essential because it aimed to cripple Japan’s war-making ability and force a surrender through economic strangulation, rather than by direct invasion. This strategy reflected a calculated use of naval power to isolate, exhaust, and coerce an island nation heavily dependent on maritime trade. This element of the war plan was present from all versions of War Plan Orange, even before the war itself.

As an island nation with limited natural resources, Japan was especially vulnerable to naval blockade for many of its resources. For example, during the war oil had to be imported from Indonesia, and thus tankers were subject to submarine, air, and surface attack far from Japan’s ability to protect the tankers with air cover. The same was true for rubber and other items. Some items such as food and iron were imported from China and Korea, nations offering sea lanes harder to blockade and easier for Japan to defend…if it could obtain oil supplies.

A blockade offered an alternative route to victory: slower, but less costly in terms of allied combatant lives and capable of causing internal collapse or surrender over time. U.S. planners believed that naval blockade would contribute to civil unrest and erode support for the war effort.  It was also intended to undermine confidence in the Japanese military leadership and in the Emperor’s divine authority by demonstrating their inability to defend the homeland. The goal was to cripple Japan’s ability to sustain its navy, deploy reinforcements, and maintain its empire.

Moral View of Blockades

Moral theologians, particularly within the Catholic tradition, have historically had a nuanced and often critical view of naval blockades, especially when they affect civilian populations. While blockades can be legitimate under just war theory, their moral acceptability depends on intention, proportionality, and discrimination between combatants and non-combatants. A blockade is morally permissible only if it aims to weaken a belligerent’s military capacity or hasten a just end to war. St. Thomas Aquinas and later scholastics emphasize that war actions must be directed toward the restoration of peace and justice, not revenge or total destruction.

One of the most serious moral concerns with naval blockades is that they often fail to distinguish between supplies and goods destined for combatants and/or non-combatants. In the modern warfare of WWII it was not possible for naval blockades to distinguish between war supplies and food supplies. Merchant ships traveled under military escort in water far distant from any U.S. Naval bases. It was the role of the submarine service to prosecute the war in the Western Pacific. In other words, there was no opportunity to “board and inspect”. On December 7, 1941, just hours after the attack, the Chief of Naval Operations (CNO), Admiral Harold R. Stark, issued a directive authorizing unrestricted submarine warfare against Japan. This meant that U.S. submarines could attack any Japanese naval or merchant vessel without warning, regardless of whether it was armed or in a convoy. 

The effect of blockade – as England had experienced – was that the civilian population would be affected. The moral “principle of discrimination” was nigh on impossible to implement in the Pacific war. While intentionally starving a civilian population to pressure their government is generally considered morally impermissible, in the case of Japan in WWII it was always likely that blockade would be inevitable and likely lead to civilian deaths. The question would always be one of proportionality. The estimates would always be assumptions and best estimates.

The grim reality of Japanese civilian deaths will not be something that Allied forces in the Pacific would have to face until the 1944 invasion of Saipan. By that point in the war the breadth of civilian casualties at the hands of the Japanese military was understood, at least in broad terms. 

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Image credit: various photographs from Naval Aviation Museum, National World War II Museum, and US Navy Archives.