Harbouring asymmetry
What the sinking of USNS Card teaches us about irregular maritime warfare in the Indo-Pacific
On 2nd May 1964, two Viet Cong divers slipped into the murky waters of Saigon harbour and attached Soviet-supplied limpet mines to the hull of the USNS Card. The resulting explosion sank the Second World War-era escort carrier, which was then operating as a non-combatant under the Military Sea Transportation Service. Though the USNS Card was later refloated and repaired – and no United States (US) personnel were killed – the incident had an outsized impact.
This seemingly minor tactical victory sent shockwaves through American political and military leadership. It exposed critical vulnerabilities in rear-area operations and foreshadowed the asymmetric tactics which would define much of the Vietnam War. More than six decades later, the sinking of the USNS Card deserves renewed scrutiny – especially as sub-threshold conflicts and irregular maritime threats intensify in the Indo-Pacific.
Irregular warfare at the water’s edge
The attack on the USNS Card highlights an enduring lesson: ports are battlespaces. Harbours – often assumed to be sanctuaries – are in fact chokepoints where disruption can yield enormous strategic value.
The Viet Cong saboteurs exploited local knowledge, worked as port labourers and used the cluttered, permissive urban environment to conceal their movements. They studied tides, patrol schedules and harbour routines, entering the water at night and planting explosives beneath the ship’s waterline. Harbour security mistook them for maintenance personnel.
Today, the navies of free and open nations depend on a sprawling network of Indo-Pacific ports – Yokosuka, Sasebo, Subic Bay, Guam, Darwin, Singapore and others. These facilities are essential to sustaining operations across the ‘tyranny of distance’ in the region. But they also represent soft targets for irregular forces, proxy actors and non-state threats operating in the shadows of civilian infrastructure.
Strategic shock from a tactical pinprick
While the USNS Card was quickly salvaged, the symbolic damage was far greater. The attack dominated US headlines and embarrassed both American and South Vietnamese leadership. It demonstrated – years before the Tet Offensive – that the Viet Cong could strike anywhere, even in secure areas.
This lesson remains highly relevant. Recent attacks in the Red Sea by the Houthis, incidents involving tankers in the Strait of Hormuz, drone usage and cyber strikes on critical infrastructure show how irregular maritime attacks can ripple far beyond their tactical scope. In an interconnected world, a small explosive or drone can create disproportionate strategic disruption, shaking global supply chains, raising insurance costs or forcing political recalibration.
Sub-threshold maritime actors do not need to sink ships to succeed. They just need to sow uncertainty, delay and doubt.
Technology, access and asymmetry
The USNS Card was the only major US ship sunk during the Vietnam War. It was not done by a missile or by fleet engagement, but by two men with mines and a plan. Such asymmetry should give modern force planners pause.
Today, state-sponsored proxies, maritime militias or special operations forces could replicate and amplify this kind of attack using off-the-shelf drones, uncrewed underwater vehicles or cyber tools, as seen regularly in Ukraine. The Indo-Pacific’s dense commercial traffic, sprawling archipelagos and crowded port facilities offer both concealment and plausible deniability.
The USNS Card attack was not about technological superiority by the Viet Cong. It exploited weaknesses in the harbour geography, insider knowledge and lax security. For free and open nations’ navies operating in allied or partner ports, human intelligence, cultural fluency and host nation cooperation will be just as important as sonar and steel. Much of this requires strong diplomatic – as well as military – connections.
Logistics: A tempting target
The Indo-Pacific theatre is defined by its distance and logistics demands. In any future contingency – especially around Taiwan, the South China Sea or the First Island Chain – success will hinge on ships like those operated by Military Sealift Command (MSC). These vessels, while essential, often lack the protection of combatants.
This creates a dilemma: high-value logistical ships must operate in vulnerable environments with limited defence. The USNS Card attack underscores the need to rethink force protection; not just at sea, but also in port.
Solutions include expanded underwater surveillance, uncrewed harbour patrol platforms, biometric access control and tighter coordination with host nation security services. Equally vital is resilience. Damaged assets must be replaced. Infrastructure must be redundant. And the system must be able to absorb shocks without mission failure.
Key lessons for modern irregular maritime conflict
The sinking of the USNS Card offers a valuable template for understanding irregular maritime threats. It reminds us that:
Ports are not safe zones: They are attractive targets for adversaries who cannot contest the open sea but can strike with low-cost, high-impact methods.
Irregular actors exploit sub-threshold spaces: These threats operate between peace and war; civilian and military; front and rear. The USNS Card attackers were indistinguishable from dockworkers because they were dockworkers.
Information is power: Insider access and human intelligence enabled the USNS Card attack more than any weapons system. Today, digital surveillance and cyber penetration make such access even easier – and more dangerous.
Force protection must evolve: Naval planners should shift focus from traditional high-end threats to layered defence strategies which consider sabotage, insider threats and unconventional attacks in logistical hubs.
Policy recommendations
To prepare for irregular maritime conflict in the Indo-Pacific, the navies of free and open nations and their allies and partners should take several steps:
Rethink protection for non-combatant vessels: MSC ships, prepositioned assets and other logistics enablers must be treated as high-value targets.
Expand training in littoral urban environments: Harbours and coastal cities will be contested zones, both in a physical and informational sense.
Invest in affordable port defences: From underwater sensors to drone patrols, cost-effective solutions can offer early warning and deterrence.
Deepen host nation security partnerships: Shared port security protocols, intelligence fusion and joint exercises are essential to reducing vulnerability.
The sinking of the USNS Card may seem like a footnote in naval history, but it holds enduring relevance to operations today. Failures teach that naval superiority is not just about fleets and firepower, but about defending the gaps: the spaces between domains, behind the lines, and beneath the waterline. In today’s Indo-Pacific, those gaps are wider than ever – and adversaries are watching.
Edward Salo, PhD, is Professor of History and Chair of the History Department at Arkansas State University. He is a former Fellow of the Modern War Institute at West Point and the Joint Special Operations University, as well as a former Civilian Aide to the Secretary of the Army. He also served as a member of New America’s Nuclear Security Futures Group.
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