In practical materiel terms, the single greatest factor in bringing about Japan’s defeat during World War II was the destruction of the Japanese merchant fleet

 

In practical materiel terms, the single greatest factor in bringing about Japan’s defeat during World War II was the destruction of the Japanese merchant fleet. 

This was critically important because Japan, like Britain, was an island nation dependent upon maritime commerce for its very survival. 

However, unlike the British, the Japanese were far less adept at defending their lines of communication. First, Japan started the war with a merchant fleet that was only a third of the size of Britain’s containing just under six million tons of shipping. Similarly, the Imperial Japanese Navy was severely delinquent in defensive assets. This was in keeping with Japanese doctrine that was offensive in nature and sought to prevail in large, decisive fleet actions to bring about a quick victory. 

As hostilities progressed, the Japanese remained stubbornly negligent in addressing their defensive deficiencies and initially failed to implement rudimentary defensive measures including the introduction of a comprehensive convoy system. The Allies were quick to take advantage of these shortcomings, and by the end of 1943 Japanese merchant shipping losses numbered almost three million tons of which nearly two thirds were sunk by American submarines. Making matters worse, this rate of attrition was accelerating as nearly 30 percent of these total losses occurred in the last four months of this period.

Given this ongoing carnage, the Japanese were finally forced to address their defensive deficiencies. Starting modestly in 1942 and substantially expanding thereafter, the Japanese embarked upon a major shipbuilding programme to produce escort vessels. Referred to as Kaibōkans (sea defence or coastal defence ships), these ocean-going warships were more akin to frigates or escort destroyers in the Allied navies. Displacing between 740 to 940 tons and armed with two to three 4.7-inch guns and up to 120 depth charges, the Japanese eventually produced 164 of these versatile vessels in five different variants. Beyond this, the Japanese also converted existing warship designs to perform escort functions.

 This was particularly true regarding their remaining fleet minesweepers, which had their minesweeping gear removed to accommodate depth charges. In other actions, in September 1943 the Japanese established a new entity called General Escort Command to coordinate and protect their shipping movements. At about the same time the Japanese instituted a basic convoy system, which expanded over the next several months to eventually cover their primary transit routes. By mid-1944 General Escort Command possessed over 100 ocean-going escort vessels, and most Japanese merchant ships now travelled in protected convoys.

Unfortunately for the Japanese, these actions did little to reverse the growing attrition plaguing their merchant fleet. Despite its increased numbers, General Escort Command was largely a hollow force lacking the technical, tactical and operational proficiency of its Allied counterparts. 

Nor was this force large enough to effectively counter the threat arrayed against it. To this point, Japan’s 100 assorted escort vessels compared quite feebly to the 426 assorted destroyers, frigates, sloops and corvettes used by the British just to secure their home waters in January 1945. Given these factors, the Japanese only succeeded in sinking about 50 American and Allied submarines during the course of the war.

 This was an extremely poor result compared to the 692 German U-boats attributed as sunk in direct action by the Western Allies during the European conflict. It was also an inadequate result as demonstrated by the fact that Japanese merchant shipping losses continued to mount totalling 3,892,019 tons sunk in 1944. The Americans were responsible for most of these losses, but British and Commonwealth forces made some contributions to this attrition. 

In terms of means, almost two-thirds of these merchant ships were sunk by submarines while aircraft, mines and surface warships accounted for most of the rest.

The result of this carnage was a severe withering of the Japanese merchant fleet. At the beginning of the Pacific war Japan possessed 5,996,607 tons of merchant shipping. In the succeeding three years the Japanese lost 6,836,094 tons of this shipping against a gain of 3,551,334 tons through acquisitions and new construction. 

The net result of this was a merchant fleet reduced to 2,711,847 tons by the end of 1944 thus representing a 54.8 percent reduction. This, in turn, contributed to a significant decline in Japanese imports, which dropped from a prewar annual level of 67 million tons to a mere 16 million tons in 1944. Even more devastating, oil imports dropped from 1.75 million barrels per month in August 1943 to just 360,000 barrels in July 1944. 

With this, the entire Japanese war effort suffered from a growing lack of resources that plagued both its industry and military alike. Just one example of the former was an 85.68 and 95.04 percent decline in Japanese steel and aluminum production respectively from 1943 to 1945 due largely to shortages of ore, coke and bauxite. Meanwhile, Japanese oil stockpiles dropped from some 43 million barrels at the beginning of the war to just 3.71 million barrels available on 1 April 1945.

By mid-summer 1945 the Japanese merchant fleet was a spent force having lost another 1,782,140 tons of shipping with most of its remaining vessels non-operational due to damage or fuel shortages. This collapsed Japan’s seaborne trade and severed the Japanese home islands from external support. With its economy and war industries in freefall decline, Japan surrendered shortly thereafter. Pictured here is the American submarine Paddle (top) and the British submarine Storm (bottom), both of which participated in operations against the Japanese. 

Unknown author and Royal Navy official photographer, public domain. For more information on this and other related topics, see Forgotten War, the British Empire and Commonwealth’s Epic Struggle Against Imperial Japan, 1941-1945.