Japan and Cambodia: Partners in a diplomatic balancing act

4 August 2025

In recent years, there has been heightened interest in the conduct of Global South countries amid the evolving long-term strategic rivalry between the US and China. China maintains a significant presence in Southeast Asia, including through its close relationship with Cambodia. There is growing concern about China’s influence in Cambodia, particularly considering the recent Chinese-funded expansions of the Ream Naval Base and the Sihanoukville Port. At the same time, Cambodia remains strongly influenced by the US, as demonstrated by the Trump administration’s initial threat of a 49 per cent reciprocal tariff on Cambodia, which was subsequently reduced to 19 per cent following negotiations. While the US tariffs partly reflect an effort to bypass Chinese exports, it also highlights the Cambodian economy’s deep integration with those of developed countries.

Like many developing countries, Cambodia avoids a binary foreign policy stance that would require it to align exclusively with either the US or China. Rather, it seeks to maximise national interests by balancing relations with multiple partners. While there are variations in how developing countries navigate their foreign relations, it is important to consider these countries as key actors rather than passive arenas of competition. Cambodia’s Prime Minister Hun Manet has shown a consistent awareness of the need for balanced diplomacy, which is reflected in his government’s policies.

Cambodia’s balanced diplomacy: Ream Naval Base expansion

As a small country, Cambodia must promote national unity and assert its autonomy while navigating competition with regional powers such as Thailand and Vietnam. The recent expansion of the Ream Naval Base, strategically located on the Gulf of Thailand, is intended to act as a counterbalance to these regional powers. However, lacking the financial resources to support the naval base expansion independently, Cambodia turned to foreign assistance — namely from China. Yet, accepting Chinese support risks deepening perceptions of overdependence. To navigate these competing pressures, Cambodia must engage in carefully balanced diplomacy that affirms its autonomy while leveraging external partnerships.

Japan and other US allies must strengthen ties with Southeast Asian countries to address US retrenchment, positioning themselves as credible alternatives for countries seeking to avoid over-reliance on China.

Cambodia’s balancing act carries significant risks, including concerns that China might use the Ream Naval Base expansion to advance its own strategic interests. Situated on the Gulf of Thailand, the naval base has sparked regional unease over a potential Chinese military foothold. Although Prime Minister Hun Manet and former Prime Minister Hun Sen have repeatedly denied that the naval base would be used regularly by the Chinese Navy, Chinese vessels have nonetheless docked there for military training. In May 2024, the Cambodian Navy and the Chinese People’s Liberation Army Navy conducted Golden Dragon 2024, a joint exercise with the objective of exchanging information and experiences, developing training capabilities, and testing and sharing new technologies. These developments have only reinforced unease over Cambodia’s increasing military alignment with Beijing.

Despite significant Chinese involvement in the expansion of Ream Naval Base, the first foreign vessels to enter the naval base after construction was completed in April 2025 were from Japan’s Maritime Self-Defence Force. Japan — a historical ally of Cambodia — played an important role in Cambodia’s peace process during the early 1990s. Although less influential than China, Japan remains a key strategic partner for Cambodia, serving as a counterweight to Beijing. By dispatching minesweepers Bungo and Etajima as the first foreign vessels to enter Ream Naval Base, rather than destroyers, Japan likely aimed to underscore its commitment to freedom of navigation, in line with its vision for a “Free and Open Indo-Pacific (FOIP)”.

Japan's FOIP vision consists of three pillars: promoting and establishing the rule of law, freedom of navigation, and free trade; pursuing economic prosperity through improving physical, institutional, and people-to-people connectivity; and maintaining peace and stability through developing maritime law enforcement capabilities, humanitarian assistance, and disaster relief. While Japan’s FOIP vision does not explicitly reference liberal democracy, it consistently promotes freedom through core liberal values such as freedom of navigation, rule of law, and free trade. In doing so, Japan likely seeks to signal its pursuit of regional stability rather than heightened military tensions.

Japan’s approach in Southeast Asia

Japan values its role — alongside Vietnam — as a counterweight to China in Cambodia’s balanced diplomacy strategy. Japan has become a trusted partner through its contributions to peace building and development in Southeast Asia, exemplified by its involvement in Cambodia’s peace process during the early 1990s. Its alliance with the US has enabled it to maintain a strategic balance with China while avoiding direct conflict. Tokyo must continue to be seen as a viable strategic partner by Cambodia and other Southeast Asian nations.

Japan's diplomatic engagement with ASEAN is based on several principles, including ASEAN centrality and Southeast Asia’s geopolitical significance. Situated between the Pacific and Indian Oceans, Southeast Asia lies at the heart of Japan’s FOIP vision. China’s ongoing maritime expansion has already crossed the First Island Chain and is extending into the Second Island Chain, with increased military presence in the East China Sea, South China Sea, and Taiwan Strait. During the second Trump administration, China has deepened its ties with the Global South, particularly with Southeast Asian countries, which rank among its largest trading partners.

At the 50th anniversary of ASEAN-Japan Friendship and Cooperation in December 2023, both sides emphasised the need to renew and strengthen their relationship, driven by concerns over China’s growing influence among ASEAN countries. To mark the 50th anniversary, the Japanese Ministry of Foreign Affairs convened an expert panel that recommended deepening Japan-ASEAN ties as equal partners, with a focus on ASEAN centrality and the ASEAN Outlook on the Indo-Pacific (AOIP).

Accordingly, Japan should refrain from framing its engagement with ASEAN through the lens of value-based competition with China or the US-China strategic rivalry. Rather than focusing on differences in political values, Japan should adopt a pragmatic approach that accounts for ASEAN’s diverse domestic contexts, religious traditions, and policy priorities. Most Southeast Asian countries — except those with direct sovereignty disputes with China, such as the Philippines — are wary of escalating military tensions and do not want to be pressured into adopting standards set by developed countries, particularly in the realm of economic security. It must be noted that while issues such as energy, food security, and the supply chains of essential goods are widely discussed, relatively limited attention is paid to managing dual-use advanced technologies.

The comprehensive strategic partnership between Japan and Cambodia

At the 70th anniversary of their diplomatic relations in 2023, Japan and Cambodia formalised their bilateral relationship as a “comprehensive strategic partnership”. Atsushi Ueno, Japan’s ambassador to Cambodia, stressed that the Japan-Cambodia relationship should be seen not as one-sided or purely bilateral, but as a partnership to address shared regional and global challenges. Under their comprehensive strategic partnership, both countries should take a more active role in promoting peace and prosperity in the region and beyond.

For Cambodia’s balanced diplomacy to succeed, Japan must remain a reliable partner. From Japan’s perspective, Cambodia is a strategic linchpin — both for securing vital sea lanes and advancing its FOIP vision. To sustain and deepen the partnership, Japan must follow through on its advocacy of the AOIP, which focuses on maritime cooperation, connectivity, the UN Sustainable Development Goals (SDGs), and ASEAN centrality. At the same time, it should avoid overemphasising the competitive and military dimensions of the US-China strategic rivalry.

As the Trump administration takes a harder line with both its allies and Global South countries — evident in major cuts to USAID funding — the role of like-minded partners has become increasingly critical. Japan and other US allies must strengthen ties with Southeast Asian countries to address US retrenchment, positioning themselves as credible alternatives for countries seeking to avoid over-reliance on China.

DISCLAIMER: All views expressed are those of the writer and do not necessarily represent that of the 9DASHLINE.com platform.

Author biography

Dr. Shin Kawashima is a Professor of International Relations at the Graduate School of Arts & Sciences at the University of Tokyo. He is the Executive Director of Research at the Nakasone Peace Institute (NPI) and a Senior Fellow at the Japanese National Security Agency. He studied Chinese/Taiwanese diplomatic history. His first book, Formation of Chinese Modern Diplomacy (2004), was awarded the Suntory Academic Prize in 2004. In 2023, he received the Japanese Foreign Minister's Commendation. Much of his current affair’s commentary can be found in The Diplomat and Think China. Image credit: Official Website of the Prime Minister’s Office of Japan (cropped).