For years, the South China Sea has been seen as a potential flashpoint in the intensifying geopolitical rivalry between China and the United States.

Southeast Asian states are often presented as the smaller powers caught in the storm.

That picture is changing, however, as a new and subtle security network takes shape across the region.

It's built not around formal alliances but around access deals, missile sales, coast guard drills, intelligence-sharing talks and defense consultations.

On June 1, the Philippines and Vietnam upgraded ties to an enhanced strategic partnership and signed a memorandum on defense cooperation which commits them to high-level exchanges, strategic dialogue, information sharing and joint activities at sea.

The partnership builds on earlier coast guard arrangements, including a hotline and mechanisms to prevent incidents.

A raft of defense deals
Late last month, Indian officials said that New Delhi had signed a $629 million (€555 million) deal to supply Vietnam with the BrahMos supersonic cruise missile system.

Vietnam is the second Southeast Asian buyer publicly confirmed by India, after the Philippines, which signed a $375 million contract in 2022 for three BrahMos batteries. The missiles are jointly developed by India and Russia.

Meanwhile, Japan's security relationship with the Philippines has grown quite concrete. A Reciprocal Access Agreement entered into force last September, expanding troop deployment in each country, while this month they began talks over a new deal to share classified defense information.

In February, Australia and Indonesia signed the "Jakarta Treaty," a common security pact that commits the two governments to regular top-level consultations.

A not-quite united front against China
None of these agreements creates a NATO-style alliance. Together, however, they show how middle powers are trying to make unilateral pressure at sea harder and costlier.

All these countries share concerns about China and an interest in upholding the rule of law at sea and keeping the region free from coercion by great powers, Hunter Marston, director of the Southeast Asia Program at the Lowy Institute, told DW.

At the same time, they recognize — to varying degrees — the possibility that the US could scale back its commitments to regional security, which would introduce volatility and unpredictability into the prevailing balance of power, he said.

"While they don't all see eye to eye on regional security or support the United States military presence equally, now is the time for middle powers to band together to stand up for their interests. None are strong enough to do so alone," Marston added.

"Even Indonesia and India, major powers in their own right, are looking to one another to shore up their security."

The rise of multilateral deterrence
The Philippines has moved the fastest. Since President Ferdinand Marcos Jr. entered office in 2022, Manila has become more willing to publicize Chinese pressure in the South China Sea and to widen defense cooperation beyond its main ally, the United States.

Manila and Hanoi still have overlapping maritime claims of their own, yet both face Chinese pressure in the South China Sea and share an interest in preventing disputes from being settled by force or intimidation.

Their upgrading of relations this month reflects that calculation. In 2024, their coast guards held their first joint exercise. This month's agreements go even further, linking defense cooperation to maritime law, humanitarian assistance, counterterrorism and peacekeeping.

This upgrade "reaffirms Vietnam's unique and enduring position as the sole strategic partner of the Philippines in Southeast Asia," Marcos Jr. was quoted as saying at the signing ceremony in Manila.

Preserving strategic autonomy
India's role, on the other hand, is different. New Delhi is not a South China Sea claimant, and its rivalry with China is focused on the Himalayan border and parts of the Indian Ocean. But the BrahMos deal gives its "Act East” policy a harder edge.

For Hanoi, the missile system strengthens coastal defense and complicates any naval or coast guard operations near its shores. For New Delhi, it shows that its Indo-Pacific policy is no longer only about diplomacy and trade. It is also about defense exports, maritime security and strategic signaling.

Despite Vietnam's public disputes with China, particularly in the maritime domain, Hanoi remains cautious, seeking to preserve strategic autonomy and avoid formal alignment, Kei Koga, associate professor at the Public Policy and Global Affairs Programme at Nanyang Technological University, told DW.

"India's BrahMos deal with Vietnam fits this pattern, as it strengthens Vietnam's own deterrence and coastal defense capacity without making Vietnam part of an anti-China alliance,” he added.

Indonesia is more cautious still. Jakarta does not describe itself as being in a territorial dispute with Beijing. But China's expansive claims overlap with waters around the Natuna Islands, and Indonesian officials know the problem is more than just theoretical.

Japan and Australia join the equation
Tokyo was once far more cautious about security cooperation in Southeast Asia, given constitutional limits on what its military can do. That is now changing, however.

The Japan-Philippines Reciprocal Access Agreement was the first of its kind that Tokyo concluded with a Southeast Asian country. Tokyo and Manila began talks this month on a General Security of Military Information Agreement, which would allow the secure exchange of classified defense information and could ease future transfers of Japanese military equipment.

"The Japan-Philippines Reciprocal Access Agreement is a notable example because it makes defense cooperation more operational, not just symbolic,” said Koga.

Australia's Jakarta Treaty with Indonesia, signed in February, points in the same direction. It is not a mutual defense treaty and does not oblige either side to fight for the other. But it creates a habit of consultation at the top of government and gives both sides a framework for cooperation if regional conditions deteriorate.

Trust in US fading
China remains the largest trading partner of most Southeast Asian countries and a vital economic partner for Japan, Australia and India, making regional players wary of antagonizing Beijing.

These expanding defense partnerships "are still loose, flexible, and uneven, with no integrated command structure or formal collective-defense commitment, as those frameworks do not demand any obligations or commitments,” said Koga.

They are "primarily about strengthening each state's own capabilities, rather than creating a system in which they rely on others for their national defense.”

Also, the recent deals signal that "the spokes are increasingly unified in their threat perceptions and willingness to put up the money to match it,” said Marston.

He added that the whole region is increasingly skeptical of the United States' reliability. Therefore, "we are witnessing a more decentralized and multilateral approach to security, which in the long run is probably a good thing.”

The South China Sea story has not stopped being about China and the United States. But it is now also about middle powers learning to work around the limits of both.