Imperial Gamble: Lessons from Japanese Sea Denial during the Battle of Okinawa

Deep in the Western Pacific, the battle rages. Loitering munitions circle overhead, each waiting its turn to dive into the enemy fleet below. One-way attack boats swarm through green waters, their low profiles obscured by wavetops in the waning daylight. In every domain, the combatants employ disruptive new technologies and tactics alongside legacy capabilities and concepts. No, this battle is not for Taiwan in 2030. The year is 1945, and an Allied naval task force is seizing the island of Okinawa against a ferocious Japanese defense. 

The Battle of Okinawa was a crushing lesson in superlatives for both the Allied and Japanese combatants. From March to June 1945, the seas around the Ryukyu Islands watched impassively as one of the largest naval task forces ever assembled collided with an existentially resolute Japanese defense. The clash proved to be one of the costliest battles of the Second World War in terms of human lives. The battle culminated in the seizure of Okinawa, which set conditions for an Allied invasion of the Japanese home islands that was forestalled only by the nuclear bombings of Hiroshima and Nagasaki. 

During the Battle of Okinawa, Japanese forces employed novel sea denial technologies—namely the kamikaze—that radically challenged the Allied fleet’s sea control and inflicted heavily asymmetric casualties. This article examines how Japan employed these capabilities to contest control of the sea against a technologically and materially superior Allied fleet. The United States should derive lessons from this battle to prepare for a potential cross-strait invasion of Taiwan by the Chinese People’s Liberation Army Navy (PLAN). 

Allied Approach

The Allied mission from operational planning documents rings clear, calling for the immediate capture, occupation, and defense of Okinawa and the establishment of sea and air control in the area.¹ U.S. Navy leaders fully understood the threat to the fleet from traditional Japanese aviation tactics and had learned hard lessons from early kamikaze attacks in 1944. The pre-battle Joint Staff Study noted, “Our expeditionary forces will be subject to strong attacks by the Japanese aviation staged through Kyushu or the China Coast and Formosa.”² Another intelligence estimate noted, “The enemy in all probability will continue the use of suicide bombing tactics against shipping and supplies. Formosa and Southern Kyushu are within operational range for all major Japanese aircraft types, including single-engined fighters.”³

With this threat in mind, allied planners sought to counter the Japanese air threat prior to the main invasion of Okinawa. A fast carrier task force would launch from Ulithi ahead of the invasion fleet to conduct shaping operations in the form of strikes on Japanese airfields on Formosa, Okinawa, the Southwest Islands, Kyushu, and the mainland.⁴ Following the fast carrier attacks on all three locations, the US Navy would conduct a battle turnover of fires to the US Army Air Corps, who would “initiate attacks on enemy air bases in Formosa as soon as the situation on Luzon permits and will maintain neutralization of the airfields in Formosa following the carrier attacks on these areas.”⁵ As it happened, the US naval shaping strikes on mainland Japan would mark the start of a Japanese sea-denial campaign prosecuted by land-based air strikes and kamikaze attacks that would only accelerate as the battle for control of Okinawa progressed. 

Japanese Air and Kamikaze Attacks

By the beginning of the Okinawa campaign, three years of combat attrition had greatly diminished Japanese aviation capabilities. With its air combat power significantly depleted, and facing a severe shortage of experienced aviators as well as shortened timelines for training new pilots, Japan adopted a strategy that preserved its most experienced pilots and units for defense of the homeland.⁷ The kamikaze emerged as a natural consequence of these constraints in men and materiel. New Japanese replacement pilots were not fully trained in the fundamentals necessary to conduct precision dive-bombing and torpedo attacks against surface targets.⁸ By comparison, kamikaze attacks required a significantly lower level of technical skill and task load. Pilots could focus entirely on guiding their payload to the target. Moreover, committing to one-way attack tactics effectively doubled the combat radius of Japanese aircraft, and the body-crashing tactics required no specific platform or payload. Together, these attributes conferred to Japan a profoundly disruptive aviation capability that could inflict considerable asymmetric effects on the Allied fleet at a time when overall Japanese combat power was rapidly waning. Ignoring the social and cultural implications of suicide attacks, any military leader would accept such a wartime tactical innovation. Ian W. Toll succinctly captured the essence of Kamikaze attacks: "The suicide plane was like a weapon from the future, allowing the Japanese to deploy guided missiles at a time when no other combatant possessed such weapons, or effective measures to counter them. " Kamikaze attacks would form an integral part of Japan’s sea denial strategy during the Okinawa campaign. 

Figure 2 uses data derived from Samuel Eliot Morison’s Victory in the Pacific to show the effectiveness of Japanese attacks on the Allied fleet during the Battle of Okinawa.⁹ In March 1945, shaping operations in the form of Allied carrier-based airstrikes on Japanese air bases on the mainland took an inauspicious turn. Beginning on 18 March, Japan’s Fifth Air Fleet launched massed air combat sorties against allied carriers loitering in the waters near Kyushu. The following day, aided by weather unfavorable to air defense radar and Allied combat air patrols, the Japanese struck the fleet carriers USS Wasp (CV-18) and USS Franklin (CV-13) using traditional dive-bombing techniques, resulting in more than 1,300 Allied casualties. The Franklin was so severely damaged that it ceased operations and limped back to the United States, never to see combat again.¹⁰ However, as shown in Figure 2, these strikes marked the final time traditional Japanese air power would be used to such effect in the Battle of Okinawa. They were anomalies in a fight soon to be dominated by the kamikaze.

The Japanese had conducted kamikaze attacks intermittently throughout 1944, but the Battle of Okinawa was defined in part by the volume and intensity of such attacks. From April 1945 onward, kamikaze attacks produced most afloat Allied casualties. The asymmetric nature of kamikaze tactics also compelled the Allied naval task group to constantly adjust its air defense tactics to protect its fleet and escort carriers, at the cost of resources, operational efficiencies, and lives. Cheerful and colorful names for Allied air-defense tactics such as the “Moose Trap” and the “Big Blue Blanket” masked a deadly reality.¹¹ The building block for fleet air defenses was the radar-picket destroyer, placed at the edge of naval formations along the axes of expected Japanese air attacks. The picket destroyers would serve as an early warning system and a screen for the mass of the naval task force. These vessels would use radar to direct combat air-patrol fighters to interdict kamikaze raids, or attempt to destroy enemy aircraft through direct fire. But in extremis, they would offer their hulls as targets for kamikazes to spare the critical naval capabilities deep in the formation.¹² 

Figure 3 again uses data derived from Victory in the Pacific to demonstrate the effectiveness of different Japanese attack methods on various Allied fleet vessels.^{13, 14} Kamikaze attacks produced an outsized majority of mission and catastrophic kills on the Allied fleet, with Allied destroyers—including destroyer-class minelayers, minesweepers, and escorts—absorbing the preponderance of the effective strikes. Kamikaze tactics ultimately created an asymmetric dilemma the Allied fleet could not ignore.  Single-pilot aircraft liberated from the requirements of survival could hold warships at risk in unprecedented ways. Radar picket tactics and disproportionate destroyer losses were the answer to this inescapable dilemma. 

Additional One-Way Attack Technologies

Before 1945, the Japanese began to produce a wide portfolio of manned one-way attack weapons for sea denial. Resembling a modern air-launched antiship cruise missile, the Oka would drop from Japanese bombers and attack surface vessels, relying on rocket propulsion under human control.¹⁵ Engineers developed similar capabilities for the sub-surface domain that resembled manned submarine-launched torpedoes. Likewise, a surface capability called the Shinyo, a manned plywood speedboat platform with an explosive payload, would target Allied vessels with crash attacks.¹⁶ 

For various reasons, these capabilities either did not see wide employment at the Battle of Okinawa or were less effective than the kamikaze. For example, for a brief period in 1944, the Japanese Navy possessed the world’s largest aircraft carrier, the Shinano, converted mid-production from a superbattleship hull. In an exceptionally inauspicious turn, after departing Tokyo on its maiden voyage 28 November, the Shinano was immediately tracked and sunk by an American submarine, taking its complement of fifty Oka one-way attack missiles and six Shinyo speedboats to the black.¹⁷ In a similar turn of Japanese misfortune, the Allied seizure of the Kerama islands in preparation for landings on Okinawa uncovered 350 Shinyo speedboats.¹⁸ While the remaining suicide boats achieved minor effects against the Allied fleet, with five attacks striking U.S. Navy ships, inflicting 29 casualties and taking three vessels out of action,  they were far less effective than kamikaze attacks.¹⁹ The Allies also profited from oversights in the design of the Oka weapon system. The capability’s short range limited the standoff distance at which it could be launched, requiring its delivery platform, the relatively slow and targetable G4M bomber, to close with Allied air defenses.²⁰ 

During the Battle of Okinawa, one-way attack tactics arguably extended to traditional Japanese surface combatants. During the Ten Go operation on 6-7April, a Japanese surface contingent including the superbattleship Yamato sortied from Tokuyama with orders to careen into the Allied fleet off the coast of Okinawa, ostensibly as a suicidal feint meant to distract and overwhelm the Allied amphibious force as it faced simultaneous massed air and kamikaze strikes and a ground counterattack ashore.²¹ As with the Shinano, the Yamato and its contingent were tracked immediately upon departure and decimated, in this case by Allied air power.²² This finale for the Yamato and its escorts effectively ended the relevance of the Japanese surface fleet, which was now unable to effectively conduct sea denial or contest Allied landings on mainland Japan.²³ For the remainder of the Battle of Okinawa, effects on the Allied fleet were left primarily to the kamikaze. 

Battle Post-mortem and Lessons for Taiwan

A combination of design flaws, material shortages, operational planning mistakes, and bad luck hampered Japan’s use of its one-way attack portfolio to effectively prosecute a sea-denial campaign during the Battle of Okinawa. In addition, while this discussion has considered Japan’s one-way attack capabilities in the context of a deliberate sea-denial campaign, Imperial Japanese Navy doctrine before World War II focused on Kantai Kessen, or the decisive naval battle. Naval engagements at the Philippine Sea and Leyte Gulf in 1944 showed the Imperial Navy’s continued commitment to this idea. It is unclear whether the development of one-way attack capabilities, tactics, and their subsequent use as the primary means of resistance during the battle for sea control around Okinawa denoted a shift in thinking, or even the adoption of a doctrinally defined operational approach. It seems likely that these capabilities and tactics were simply the last means of self-defense available to a Japanese state incapable of comprehending defeat.²⁴ Had Imperial Japan been able to address the problems mentioned above, it might have waged a more effective sea-denial campaign. How the Battle of Okinawa might have unfolded had Japan been able to effectively employ its entire range of one-way attack capabilities is forever left to speculation.

Such speculation could be instructive when considering the current strategic issues facing U.S. naval forces in the Indo-Pacific. If China tries to invade Taiwan, the contest for control of the sea in the western Pacific may in some ways resemble the Battle of Okinawa. The problems the Navy and Marine Corps face might be recognizable to a Japanese naval officer in 1945. For example, the PLAN would likely attempt to establish sea control to isolate Taiwan, aided by a significant regional firepower advantage and a high number and concentration of naval capabilities. Meanwhile, the preponderance of U.S. combat power will be unable to persist in the littorals under a hostile and sophisticated sensing and targeting regime. Committing U.S. capital ships to a fight in the waters around Taiwan without mitigating PLAN advantages would be akin to the reckless and catastrophic employment of the Yamato during Ten Go. 

Nonetheless, Japan’s aggressive one-way attack tactics during the Battle of Okinawa help show how to hold a superior naval force at risk under otherwise unfavorable circumstances. First, as with the Japanese Imperial Navy, the United States is unlikely to achieve uncontested sea control near Taiwan for any considerable length of time, given the significant regional advantages of the adversary. The United States and its allies must therefore possess a theory of victory, supported by applicable doctrine, that does not rely on decisive sea control—something the Imperial Japanese lacked. 

Second, human and cultural considerations aside, Japan’s use of distributed, one-way attack tactics—particularly the kamikaze—exemplified a textbook use of constrained capital and material resources to inflict disproportionately lethal effects on a technologically and numerically superior adversary. The successful aerial and surface drone attacks of the past two years in Ukraine and the Black Sea show such capabilities and tactics are still relevant. Military decision-makers across the Sea Services should carefully consider how well the current portfolio of littoral fires capabilities aligns with this paradigm. 

Finally, magazine depth in any engagement in the western Pacific will be critical to victory. Japanese aviators and kamikaze pilots flew more than 3,700 sorties to achieve lethal effects on the Allied fleet at Okinawa.²⁶ Asymmetric naval capabilities must be cheap and plentiful.

Reflecting on the Allied plan for the capture of Okinawa (Figure 4), it is hard not to contemplate how much and how little has changed in nearly eight decades. The politics and alliances have shifted, but the geography and people are the same, and if history does not directly reflect across time, it may very well rhyme. 

Majors Barlow, Harper, and Reilly are operations research analysts at the Marine Corps Operations Analysis Directorate. They are also co-founders of War Quants, a publication bringing data-driven insights to national security topics.