Defence Of Narrow Seas In Southeast Asia – Analysis


Southeast Asia is in the midst of an unprecedented level of naval modernisation. Given that Southeast Asian waters constitute both enclosed and semi-enclosed seas, how do these geographical conditions affect maritime defence?

By Ristian Atriandi Supriyanto

SOUTHEAST Asian states are pursuing an unprecedented level of naval modernisation as tensions in the South China Sea test their abilities to find diplomatic solutions to their territorial disputes. According to projections by a US-based naval consulting company, AMI International, Southeast Asia is set to spend more than US$25 billion on new naval acquisitions through 2030, with the core of future projects to include fast attack craft and submarines. However, the configuration of Southeast Asian seas as enclosed and semi-enclosed seas (“narrow seas”) could affect and influence maritime defence in the area.

Enclosed and Semi-Enclosed Sea

Southeast Asia
Southeast Asia

Enclosed and semi-enclosed seas are defined in the United Nations Convention on the Law of the Sea (UNCLOS) as “a gulf, basin or sea surrounded by two or more states and connected to another sea or the ocean by a narrow outlet or consisting entirely or primarily of the territorial sea and exclusive economic zones of two or more coastal states”. As argued by Milan Vego of the US Naval War College, enclosed sea “lies wholly within the continental shelf, and is surrounded by a landmass except for a strait or several straits that connect it to an ocean or another enclosed or semi-enclosed sea”. Semi-enclosed sea is more characterised by large tidal ranges that are oceanic in nature.

Vego defines narrow seas militarily as “bodies of water that can be controlled from both its sides”. As such, maritime forces operating in narrow seas are much less immune from the land-based weapons of coastal states than in open seas. This makes defence of, and in, narrow seas different from that of the open seas, including in Southeast Asia.

Defence of Narrow Seas

Defence of narrow seas is primarily characterised by two caveats: The lack of physical space and closer proximity to land. As Vego asserts, these would bring consequences for maritime forces in terms of their manoeuvrability, combat intensity, battlespace spectrums, and the use of airpower. In narrow seas, large surface combatants are much more difficult to manoeuver due to more restrictions in depth, width, and access.

Maritime combat operations will also be more saturated and intense since most narrow seas are relatively within the effective range of land-based weapons. It can also be multi-dimensional across all spectrums of battlespace: air and space, surface and subsurface, coastal, and electromagnetic, as the propensity to confront the whole fleet of a coastal state is much greater than in open seas.

Furthermore, narrow seas are still within the range of land-based combat aircraft which makes it relatively easier for coastal states to challenge sea control and conduct sea denial operations without actually having robust naval combat assets at sea. Sea control aims to use the sea for oneself, and if necessary, prevent the enemy from using it; whereas sea denial aims to deny the enemy’s fleet from operating in a particular sea area, but does not aim to use it for oneself.

Narrow seas often constitute the territorial seas and exclusive economic zone of a coastal state that makes control over them desirable due to social and economic necessities. It is also in narrow seas that a weaker coastal state can effectively challenge a superior maritime power using an array of “asymmetric” maritime capabilities such as naval mines, coastal missile batteries, and submarines.

Presumably, more investment toward these capabilities might be more destabilising as they could only be used for warfighting purposes. This could generate misunderstanding and miscalculation, especially when they are deployed in disputed narrow seas, like the South China Sea.

Southeast Asian Context

In Southeast Asia, there is no shortage of narrow seas to influence regional maritime defence. Historical precedents from World War II offer salutary examples of defence in narrow seas. The capital ships of the British Royal Navy, the HMS Prince of Wales and Repulse, were sunk entirely from the air for the first time in combat in the South China Sea by Japanese bombers off the east coast of Malaya. Hence, control over narrow seas often requires control over the air above and water column below.

Amidst the growing maritime competition between the key powers in the South China Sea, the coastal states seem to be gearing themselves up to meet this challenge. Submarines are already in the fleets of Indonesia, Singapore, Malaysia, and in the top shopping list of Vietnam, with Thailand and Philippines mulling acquisition too. Maritime strike aircraft, like the Sukhoi Su-30, have entered the inventories of Indonesia, Malaysia and Vietnam. The Philippines would also start procuring fighter and surface attack aircraft, apart from boosting its depleted air surveillance capability.

Vietnam has acquired two Russian Bastion coastal missile systems that could hit a ship 300 km away from the coast. For Indonesia, naval mine warfare has been designated as a crucial element of its naval strategy. More importantly, coastal states could also exploit the islands in the area by building infrastructures to support exertion of sea control over the surrounding waters. Control over maritime choke-points and passes are also crucial as they guarantee access for maritime powers to transit in regional narrow seas.

For the maritime powers, too, they need to adapt to more agile and versatile platforms, like corvettes, frigates, and littoral combat ships. Without the latter adequate escort, there is a great risk to deploy large platforms like amphibious assault ships or aircraft carriers in hostile narrow seas.

However this development should not be a cause for alarm. Regional maritime forces still have a long way to go to transform platforms into real capabilities. There are some important questions about their capacities to effectively operate them, ensure safety during their operational use, as well as quality maintenance. Unless these questions are properly addressed, one should doubt the utility of these capabilities beyond being a psychological deterrent.

Ristian Atriandi Supriyanto is a Senior Analyst with the Maritime Security Programme at the S. Rajaratnam School of International Studies (RSIS), Nanyang Technological University. He was previously a researcher at the Centre for East Asian Cooperation Studies (CEACoS), University of Indonesia.


RSIS Commentaries are intended to provide timely and, where appropriate, policy relevant background and analysis of contemporary developments. The views of the author/s are their own and do not represent the official position of the S.Rajaratnam School of International Studies (RSIS), NTU, which produces the Commentaries. For any republishing of RSIS articles, consent must be obtained from S.Rajaratnam School of International Studies (RSIS).

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