TOKYO – In a clear shift that shows Tokyo’s stronger focus on Southeast Asia, Japan is speeding up security cooperation with Vietnam and other ASEAN states. Japanese leaders present these growing ties as a shield against China’s tougher posture at sea.
As China’s activity in the South China Sea grows, Tokyo is directing billions of yen in defence support and joint drills to partners across the region. Officials say they want to help countries stand their ground, drawing clear parallels with Japan’s own tensions with China around the Senkaku Islands in the East China Sea.
This broad outreach, which combines one-to-one deals with regional initiatives, reflects a steady and focused Japanese effort to protect a rules-based order in Asia’s disputed waters.
Recent moves show how active Japan has become. In early November 2025, during talks on the sidelines of the ASEAN Summit in Malaysia, Prime Minister Sanae Takaichi promised an extra ¥50 billion (about $330 million) in official development assistance to Vietnam.
The money is set aside for maritime surveillance radars and patrol vessels. It builds on a 2023 decision to upgrade Japan-Vietnam ties to a “comprehensive strategic partnership”, a step that has already seen Tokyo hand over six high-speed coast guard ships to Hanoi. “Vietnam’s response to pressure is setting a standard for the region,” Takaichi said, hinting at shared concerns without naming China directly.
Vietnam’s Long Confrontation
Vietnam’s security partnership with Japan rests on its long-running disputes with China over the Paracel and Spratly islands. These hotspots have been a source of tension since the 1970s. The Paracels, called Hoang Sa in Vietnam, were seized by Chinese forces in a 1974 naval battle that killed more than 70 Vietnamese sailors. Beijing still claims this was the recovery of “inherent territory”.
Hanoi counters that it has historical rights going back to the 17th century, supported by colonial-era charts and submissions under the UN Convention on the Law of the Sea (UNCLOS). The Spratlys, or Truong Sa, are even more complex. Vietnam controls around 25 features, but China has turned seven reefs into fortified islands, with runways and missile systems, that push into Vietnam’s exclusive economic zone (EEZ).
These clashes over rocks and reefs affect everyday life. They threaten Vietnam’s fishing industry and offshore energy projects. In May 2024, Hanoi condemned the presence of a Chinese hospital ship in the Paracels as a breach of sovereignty. Chinese coast guard ships have rammed Vietnamese fishing boats and used water cannons around reefs near the Spratlys.
Vietnam has answered with its policy of “cooperating and struggling”. It keeps strong trade ties with China, with two-way trade reaching $200 billion in 2024, while still upgrading its armed forces. Disappointed by the slow progress of ASEAN talks on a Code of Conduct (COC) for the South China Sea, Hanoi has looked more to outside partners like Japan for backing. As one Vietnamese diplomat put it privately, “Japanese ships are more than just equipment, they show that force will meet resistance.”
China’s Grey-Zone Strategy
China’s approach in the South China Sea breaks with international rulings and has turned a key shipping route, which carries an estimated $3.4 trillion in trade each year, into a heavily controlled space. Beijing’s “nine-dash line”, expanded to ten dashes in 2023 to loop in Taiwan and much of the Spratly chain, lays claim to about 90 percent of the sea.
It overlaps with the EEZs of Brunei, Indonesia, Malaysia, the Philippines, Taiwan, and Vietnam. The Permanent Court of Arbitration rejected these claims as unlawful in 2016, yet the map still guides Chinese policy.
It underpins a mix of island-building on reefs such as Fiery Cross, where dredgers have created around 3,200 acres of facilities with runways and radars, “maritime militia” fishing fleets that follow rival boats, and aggressive coast guard tactics like ramming. In June 2025, Chinese vessels ran aground near Thitu Island, which is held by the Philippines, and Manila raised its alert levels.
These grey-zone tactics fall short of open war but still apply heavy pressure. They chip away at other states’ control, one small move at a time. In 2025 alone, Chinese ships have tailed Philippine resupply missions to Second Thomas Shoal around 50 times and have fired water cannons that injured crew members.
Analysts at Eurasia Review call this “strategic escalation”, pointing to live-fire exercises and formations of helicopters that strengthen China’s position. For Vietnam, which has an EEZ believed to hold around 11 billion barrels of oil and gas, such actions threaten key future revenue. At the same time, China’s charm offensive runs in parallel. Around $100 billion of Belt and Road funding has flowed into ASEAN states. That mix of loans, projects, and pressure splits the group, as countries such as Indonesia and Malaysia try to balance economic gains with security concerns.
Japan’s Own Disputed Waters
Japan’s turn toward ASEAN security is not an act of simple goodwill. It reflects Tokyo’s own problems with China in the East China Sea. The Senkaku Islands, which Japan has controlled since 1895, are claimed by Beijing as Diaoyu Dao. China challenges Japan’s claim, citing maps from the 14th century.
Tokyo replies that it had around 75 years of calm control, until oil discoveries in the 1970s revived Chinese interest. Friction grew sharply in 2012 when Japan bought three of the islets from a private owner and brought them under state control. That move set off protests in Chinese cities and led to frequent Chinese coast guard patrols near the islands. Japan recorded 29 such intrusions in 2025 alone.
A clash on 2 December 2025 showed how quickly matters can heat up. Chinese patrol ships steamed close to a Japanese fishing boat near the Senkaku Islands. The Japan Coast Guard ordered them out, and both sides later claimed legal authority. Beijing called the operation a “lawful patrol”, while Tokyo listed it as the 29th illegal entry that year.
Oil and gas deposits may sit under the seabed near the islets, and the U.S.-Japan security treaty covers the islands under Article 5, which raises the risk of pulling Washington into any serious crisis.
Prime Minister Takaichi confronted President Xi Jinping about these “provocations” in October, linking the actions in the East China Sea to similar behaviour in the South China Sea. As Japan’s Ministry of Foreign Affairs states, “The Senkaku Islands are inherent Japanese territory; unilateral moves undermine regional stability.”
ASEAN as Japan’s Key Partner
Japan’s outreach across ASEAN reaches far beyond Vietnam and is shaping a broader security network. In January 2025, Tokyo agreed to provide two patrol vessels for Indonesia’s navy to help monitor the Natuna waters, where Chinese incursions rose by 40 percent in the previous year. Japan has also stepped up military cooperation with the Philippines, which is receiving Japanese radar systems and support from Japan’s new defence aid framework.
Joint drills and capacity-building led into the October ADMM-Plus meeting, where Tokyo pushed for a stronger Expanded ASEAN Maritime Forum, hoping it would influence future COC rules. These platforms grew out of Japan’s diplomatic efforts in the 1990s, such as its support for the ASEAN Regional Forum. Now they serve as tools to push back against Beijing’s divide-and-rule tactics by rooting security dialogue in UNCLOS and other shared norms.
Some policymakers in Tokyo worry about stretching Japan’s resources too far. Defence spending has risen to 2 percent of GDP, putting pressure on public finances. Supporters of the policy argue that the stakes justify the cost. They say China’s activities in the East and South China Seas form a single “coercion continuum”, a concept used in analysis by SWP Berlin.
Vietnam’s “four Bs” approach, broadening ties, binding institutions, building capabilities, and blunting assertiveness, lines up well with Japan’s idea of a “free and open Indo-Pacific”. As ASEAN, under the Philippines’ chairmanship, works toward a fresh push on the COC in 2026, Japan’s weight could make a real difference.
During a recent visit to Hanoi, Vietnamese Foreign Minister Bui Thanh Son praised Japan as a “reliable pillar” and celebrated growing coast guard cooperation. For Tokyo, these ties go beyond support for individual islands.
They are about keeping Asia from sliding into a region dominated by a single power. As Takaichi prepares for her next ASEAN tour, Japanese officials say the message will be simple: in rough seas, friends stand side by side.
