Japan’s Shift to Regional Defense: One-Theater Strategy Aims to Boost U.S. Alliance

US-Japan Defense Planning; US-ROK Space Cooperation; US-Allies Military Exercises

Apr 18, 2025

Hub Story: Japan’s One-Theater Strategy Aims to Boost US Alliance
Spoke Story: US-ROK Hold Civil Space Dialogue
Forging Regional Readiness: US Military Exercises with Indo-Pacific Allies
Alliance Insights: Recent Articles on Alliance Cooperation

Hub Story: Japan’s One-Theater Strategy Aims to Boost US Alliance

When US Secretary of Defense Pete Hegseth visited Asia, Japan proposed a new “one-theater” security concept to the United States that would treat the East China Sea, South China Sea, Korean Peninsula, and surrounding areas as a single operational theater to counter growing threats from China. Defense Minister Gen Nakatani emphasized the need for stronger regional defense cooperation among allies like the US, Australia, South Korea, and the Philippines, a proposal welcomed by US Defense Secretary Pete Hegseth. This marks an extraordinary shift for Japan, signaling a willingness to expand its defense role beyond traditional geographic limits and adopt a more integrated regional strategy.

Theater strategy is a core organizing concept for defense planning. Essentially, theater strategy is the planning and execution of military operations within a specific geographic area, known as a theater of operations. It bridges national strategic objectives and operational-level actions, aligning military resources and activities to achieve political and strategic goals within that region.

US Army War College: An Introduction to Theater Strategy and Regional Security:

Theater strategy directs military activities ranging from peacetime cooperation with other countries, to meeting potential threats through contingency planning (previously known as deliberate planning) and crisis action planning. Theater strategy organizes a theater’s forces and operational areas, and arranges the relationship among them to ensure unified action. Theater strategy also ensures adequate logistics and other support for theater activities, and synchronizes joint, multinational, and interagency operations and training. All of this maintains military unified action within a geographic region to achieve strategic goals.

Japan’s offer to Hegseth is particularly notable given the country’s traditionally restrained defense posture. While Tokyo has pledged to increase defense spending, it has historically maintained low levels of investment and fielded a force geared primarily toward homeland defense. This approach has been shaped by pacifist public sentiment and legal constraints under Japan’s constitution, which permits only strictly defensive military operations. Over the past 15 years, Japan has taken steps to reform and reinterpret its legal framework to enable a more “normal” defense posture and expanded operational flexibility. These efforts reflect a gradual shift away from strict self-defense limitations toward a more proactive security role in the region.

If Japan is serious about the proposed “one-theater” concept, it would mark a dramatic shift in how Tokyo approaches military operations. While details remain unclear, reporting suggests Japan is now willing to plan and operate beyond its immediate homeland and territorial waters—extending its focus to the East China Sea, South China Sea, Korean Peninsula, and possibly beyond. This would represent a significant boost to US war planning and operational capacity in the Indo-Pacific.

Recent actions indicate Tokyo may be committed to expanding its regional military role. Last September, a Japanese naval vessel transited the Taiwan Strait for the first time alongside Australian and New Zealand warships—an important signal of support for peace and stability in the region. Japan has also deepened security ties with the Philippines, which continues to face Chinese coercion in the South China Sea, and has worked to strengthen defense cooperation with South Korea. Just recently, the three countries held a meeting where leaders discussed enhancing space security collaboration.

US Department of State: Joint Statement on the Trilateral – United States, Japan, Republic of Korea – Meeting in Brussels:

The United States reiterated its ironclad commitments to the defense of Japan and the ROK, backed by America’s unmatched military strength, including its nuclear capabilities. All sides confirmed that US extended deterrence commitments to Japan and the ROK remain critical to the security and stability of the Korean Peninsula and the Indo-Pacific region. Japan and the ROK expressed their support for US efforts to engage the PRC and Russia on reducing nuclear risks. They reaffirmed their commitment to bolstering defense and deterrence by advancing robust security cooperation and strengthening their respective defense capabilities.

In many ways, the “one-theater” proposal may be a strategic effort to keep the Trump administration engaged in the Indo-Pacific. Despite initial declarations that the region would be a top priority, the administration’s attention has largely been consumed by crises in Ukraine, Gaza, Yemen, Greenland, and Panama. Japan may be offering new incentives to encourage Washington to refocus on Asia. While trade tensions have strained the alliance, early negotiations suggest a potential deal may be within reach.

Spoke Story: US-ROK Hold Civil Space Dialogue

The United States and South Korea held their fourth Civil Space Dialogue in Washington, D.C., reaffirming their commitment to expanding cooperation in space exploration, Earth observation, satellite navigation, and space weather. The two sides discussed joint projects including Artemis missions, lunar payloads, human spaceflight, and regulatory alignment, while also highlighting growing commercial and scientific collaboration. They agreed to meet biennially, with South Korea set to host the next dialogue in 2027.

South Korea has approved new space strategies aimed at making it one of the world’s top five space powers, with flagship projects including reusable launch vehicles, ultra-high-resolution satellites, and a space observatory at the Sun-Earth L4 point. The Korea AeroSpace Administration (KASA) is shifting its next-generation launch vehicle program toward reusability by 2035, promoting private-sector innovation, and planning major lunar exploration and satellite initiatives. To support these goals, South Korea significantly boosted its space R&D budget and announced plans to build a new national aerospace hub by 2030.

The United States and South Korea have not always worked deeply on space cooperation, in fact Washington has historically taken steps to limit South Korean space innovation.

Korean Economic Institute: US-South Korea Relations in Space: A New Era for Partnership:

Starting in the 1970s, the United States and South Korea entered into an agreement that authorized exchanging details on US missile technology while restricting the range and payloads authorized for South Korean missile launches. Although the primary motivation of these restrictions was to limit the range and destructiveness of South Korea’s missile strike systems to prevent crisis escalation, it also had the effect of restricting South Korea’s launch technology development, including prevent the use of solid fuels in rockets.”

In 2021, South Korea and the United States finally revised their agreement on sharing missile technology to allow for technology exchange to assist with launch-vehicle development. When President Yoon came into office, he outlined an ambitious plan for South Korean space expansion under the fourth Space Development Promotion Plan. The strategy articulated a path to make South Korea a globally recognized space power by landing a robot on the Moon in 2032 and on Mars in 2045 while doubling the government’s space investments by 2027.

Additionally, the South Korean government created a new agency, the Korea Aerospace Administration (KASA) to help over see and facilitate the Space Development Promotion plan.

Source: KEI: US-South Korea Relations in Space: A New Era for Partnership

Since the announcement of the new plan, the South Korean space budget has climbed and the new US-ROK dialogue suggests the two countries will work together to make more progress.

South Korea’s growing space capabilities has obvious security benefits. During a visit to South Korea, US Space Force Chief Gen. Chance Saltzman emphasized the growing role of space cooperation in the US-ROK alliance. At Osan Air Base, he highlighted the integration of US and Korean space personnel through joint exercises, shared operations, and expanded missile warning and ISR capabilities. The visit underscored both nations’ commitment to advancing space development and strengthening deterrence through deeper collaboration in the space domain. There is also the possibility of South Korean contributions to Trump’s Golden Dome initiative will require significant investment in space capabilities. Seoul should think creatively about how they can leverage their space program to offset American security costs on the Korean Peninsula.

Forging Regional Readiness: US Military Exercises with Indo-Pacific Allies

In April 2025, the United States and its Indo-Pacific allies and partners conducted a series of high-impact bilateral and multilateral exercises aimed at strengthening military interoperability, improving crisis response capabilities, and reinforcing regional security cooperation. From combined airpower drills on the Korean Peninsula to large-scale amphibious operations with India, these exercises highlight a shared commitment to a free and open Indo-Pacific. Below is a summary of key engagements across the region involving the United States, South Korea, the Philippines, India, Japan, and Guam.

In mid-April 2025, US and Republic of Korea (ROK) forces conducted a series of joint exercises to strengthen military readiness and interoperability. B-1 bombers, F-16s, and ROKAF F-35As and KF-16s trained together in combined air operations over the western ROK, demonstrating tactical integration and alliance commitment. Meanwhile, the US and ROK navies completed SALVEX Korea 2025, an intensive salvage and diving exercise that included deep-sea dives, use of advanced underwater technologies, and joint recovery missions. Simultaneously, Korean Spring Exercise 2025 focused on combined mine countermeasures, with both navies practicing mine detection, neutralization, and clearance to enhance port and sea lane protection. These exercises underscore the strength of the US-ROK alliance and their shared commitment to regional stability and a free and open Indo-Pacific.

As part of Salaknib 2025, Philippine Army soldiers from the First Scout Regiment and US Army soldiers from the 25th Infantry Division conducted a jungle tracking course on April 9 to improve jungle warfare skills and enhance bilateral interoperability. Led by experienced Philippine instructors, the training focused on tracking, camouflage detection, and ambush avoidance in challenging tropical terrain. The exercise strengthened operational coordination and supported broader efforts to boost combat readiness and alliance cooperation between the US and the Philippines.

During Exercise Tiger Triumph 2025, US and Indian forces conducted extensive joint operations in the Bay of Bengal and Andhra Pradesh, India, aimed at enhancing interoperability in humanitarian assistance, disaster response, and maritime security. Highlights included anti-submarine warfare training between US and Indian P-8 aircraft, a large-scale amphibious landing drill involving over 1,000 personnel, and data-gathering support from US Navy assets like the P-8A Poseidon. The exercise demonstrated growing operational integration, command-and-control coordination, and a shared commitment to a free and open Indo-Pacific, marking the fourth iteration of the US-India Tiger Triumph series.

On April 3, 2025, US Navy Electronic Attack Squadron VAQ-131, under Task Force 70, conducted bilateral training with Japan Maritime Self-Defense Force (JMSDF) forces south of Okinawa, following the signing of the Fujin memorandum of understanding in December 2024. The exercise enhanced joint electronic warfare capabilities, integrating JMSDF destroyers and reconnaissance aircraft with US EA-18G Growlers to strengthen interoperability in electromagnetic and information warfare. Leaders from both navies emphasized the importance of seamless coordination and battlespace awareness in crisis scenarios, underscoring growing US-Japan cooperation under the Fujin pact.

From March 24 to April 4, 2025, the US Coast Guard partnered with the National Association of State Boating Law Administrators (NASBLA) to provide intensive maritime training for local emergency responders in Guam. The courses, held at Port Authority facilities and Station Apra Harbor, focused on key skills like boat handling, navigation, and search and rescue, enhancing interoperability between the Coast Guard and local agencies. This effort strengthened interagency coordination, built operational trust, and laid the groundwork for future joint operations, including a planned mass rescue drill with the US Navy in May.

US Air Force Airmen from the 51st Fighter Wing at Osan Air Base and the 18th Wing at Kadena Air Base, along with Republic of Korea Air Force personnel, are conducting Combined Search and Rescue Training Event (CSARTE) 25-2 at Osan AB from April 7–18. The exercise aims to enhance joint CSAR capabilities and interoperability across the Indo-Pacific, involving approximately 13 aircraft types and showcasing the scalable, flexible nature of combined US-ROK personnel recovery operations.

Alliance Insights: Recent Articles on Alliance Cooperation

Japan:

South Korea:

Philippines:

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Australia:

New Zealand:

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India:

United States:

House Armed Services: Statement of Admiral Samuel Paparo

Other:

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