How did submarine warfare weaken Japan during World War II?

During World War II, the United States Navy conducted a highly successful submarine campaign that played a critical role in the defeat of Japan. By targeting the Japanese merchant marine, U.S. submarines effectively crippled Japan's ability to sustain its war effort.1

The impact of this submarine warfare included:

• Economic Strangulation: American submarines were responsible for sinking nearly 5 million tons of Japanese merchant shipping. This accounted for half of the total shipping losses suffered by Japan during the war.12
• Resource Depletion: By destroying the merchant fleet, the U.S. Navy severed Japan's vital communications and supply routes to its newly acquired territories in the East Indies and Southeast Asia, preventing the effective exploitation of resources.21
• Strategic Isolation: The loss of shipping forced the Japanese to expose their remaining naval forces to destruction in desperate, failed attempts to supply their far-flung garrisons.2

Ultimately, this naval blockade was a central factor in Japan's economic collapse, leaving the home islands exhausted of supplies and vulnerable to final defeat.1