How insurance underwriters now decide which wars can be sustained

The ability to wage war has always depended on the capacity to move materiel across water. What has changed is that a small number of firms in London, Munich, and Zurich now exercise more practical control over those movements than most naval forces. When marine insurers designate a sea lane as high-risk or withdraw cover entirely, the commercial logistics that underpin military supply chains collapse, often more quickly and more completely than any blockade could achieve.

This is not a matter of insurers taking political positions. The mechanism is structural. Shipping companies require hull and cargo insurance to secure financing, and the reinsurance market that underwrites those policies is concentrated among fewer than a dozen major players. When actuarial models flag a region as uninsurable at standard rates, the cost of moving goods through that corridor can increase several hundred per cent within days, or cover can become unavailable altogether. The effect is binary: routes remain open or they close.

The language that governs these decisions is technical and appears apolitical. Exclusion clauses in marine insurance contracts list geographic coordinates, specify vessel types, and define triggering events such as mine strikes or missile attacks. Underwriters update these clauses in response to loss data, not foreign policy. Yet the cumulative effect of these adjustments is to create invisible barriers that function much like a naval exclusion zone, but without the need for a single warship or any declaration of intent.

States have been slow to recognise the strategic vulnerability this creates. Military planners still think in terms of sea control, air superiority, and the physical interdiction of vessels. They are accustomed to adversaries that wear uniforms and operate within a legal framework that distinguishes combatants from civilians. Insurance underwriters fit neither category. They are private actors with fiduciary duties to shareholders, operating within a regulatory environment designed for commercial stability, not geopolitical contestation.

The concentration of underwriting capacity means that decisions made by a few firms ripple across the entire global shipping network. A single reinsurer adjusting its risk appetite can strand cargoes in port, delay military resupply, and force belligerents to seek alternative routes that may not exist. During periods of heightened tension, the speed at which these decisions propagate has outpaced the ability of governments to respond, either through subsidy, state-backed indemnity schemes, or diplomatic negotiation.

This dynamic has become visible in multiple theatres. Shipping through contested waters has periodically ground to a halt not because of direct military action, but because insurers reclassified those routes as war-risk zones. The reclassification triggers contractual obligations that require ship owners to obtain additional cover, often at prohibitive cost. Smaller operators, who lack the balance sheets to absorb such increases, simply refuse to sail. Larger operators demand that charterers cover the surcharge, passing the cost up the supply chain until someone declines to pay.

The asymmetry is striking. A belligerent state can marshal naval assets, declare an exclusion zone, and enforce it with lethal force, but it must also accept the political and legal consequences of doing so. An insurance syndicate can achieve a comparable effect by circulating an updated exclusion list, and it does so without triggering any international legal mechanism or diplomatic protest. The decision is presented as a matter of actuarial prudence, and it is difficult to argue that a private firm should be compelled to underwrite risks it deems commercially unacceptable.

Yet the effect on military logistics is profound. Modern expeditionary operations depend on contracted shipping, not state-owned fleets. Fuel, ammunition, spare parts, and food move on vessels chartered from commercial operators who will not sail without insurance. If underwriters withdraw cover, the supply line breaks. No amount of naval escort can substitute for an insurance certificate, because the issue is not physical security but contractual and financial exposure.

Governments have attempted to address this vulnerability through state-backed war-risk schemes, in which the public sector assumes the liabilities that private insurers will not. These programmes exist in several jurisdictions and have been activated during previous crises. But they are slow to mobilise, require legislative or executive approval, and often come with conditions that limit their scope. More importantly, they reveal the extent to which states have ceded control over a critical enabler of military power to the private sector.

The counter-argument is that this arrangement imposes a useful discipline. If the cost of insuring a shipping route becomes prohibitive, it may signal that the military operation it supports is unsustainable or that the risk of escalation is too high. In this view, the insurance market acts as a kind of circuit-breaker, forcing belligerents to reconsider operations that might otherwise spiral out of control. The mechanism is indifferent to ideology and applies symmetrically to all parties.

There is some merit to this perspective. Insurance underwriters have no interest in prolonging conflicts; their incentive is to minimise exposure and preserve capital. A market-driven constraint on military logistics might, in theory, reduce the duration or intensity of hostilities. But it also means that the ability to sustain a war effort increasingly depends on access to a cartel-like structure of reinsurance capacity, and that access is not distributed equally.

States with deep financial markets and close relationships with the London and European insurance sectors can negotiate terms, arrange state guarantees, or apply political pressure in ways that smaller or less connected governments cannot. The result is that the insurance constraint binds asymmetrically. A well-resourced actor can bypass or mitigate the problem; a less-resourced one cannot. This introduces a form of structural advantage that has little to do with military capability and everything to do with financial architecture.

The opacity of the system compounds the problem. Exclusion clauses are not published in a centralised registry. They are negotiated in private, circulated among brokers, and embedded in contracts that are not subject to public disclosure. A government may discover that its supply lines are uninsurable only when a charterer declines a contract or demands a sudden surcharge. There is no advance warning, no appeals process, and no transparency about the criteria used to make the determination.

This lack of visibility also makes it difficult to assess whether underwriters are responding solely to actuarial data or whether other considerations are influencing their decisions. Reinsurers operate in a competitive market, but they also operate within a broader political economy. They are subject to sanctions regimes, regulatory pressure, and reputational concerns. It is not implausible that a firm might adjust its risk appetite in a way that aligns with the foreign policy preferences of the jurisdictions in which it is domiciled, even if that adjustment is framed in purely commercial terms.

The strategic implications extend beyond active conflicts. The knowledge that insurance can be withdrawn creates a latent vulnerability that shapes behaviour in peacetime. States that depend on maritime trade for critical imports must consider whether their supply lines could be severed not by an adversary's navy, but by a decision taken in a London underwriting room. This consideration affects everything from stockpiling policy to the diversification of supply sources to the willingness to escalate in a crisis.

Some governments have begun to explore alternatives. There is growing interest in mutual insurance schemes, state-owned reinsurance vehicles, and regional arrangements that reduce dependence on the incumbent market structure. But these initiatives face significant obstacles. Insurance and reinsurance are industries that benefit enormously from scale and diversification. A state-backed scheme that covers only a narrow range of risks or a limited geographic area will struggle to compete with the global capacity of the established players.

There is also the question of whether states should be in the business of underwriting war risks at all. A government that insures its own military supply lines is effectively subsidising the cost of conflict, and it does so with public funds. That may be justified in cases of national defence, but it becomes more contentious when applied to expeditionary operations or to the commercial shipping that supports them. The political economy of who bears the risk, and who profits from it, is not straightforward.

What is clear is that the current arrangement has made a small number of private firms into de facto arbiters of which military operations can be logistically sustained. This was not the result of any deliberate policy choice. It emerged from the consolidation of the reinsurance industry, the globalisation of supply chains, and the increasing reliance of states on contracted rather than organic logistics. But the effect is to create a chokepoint that is both invisible and difficult to circumvent.

The question is whether this represents a useful constraint on the use of force or a dangerous transfer of strategic decision-making to actors with no public accountability. The answer likely depends on one's view of the legitimacy and effectiveness of state power, but it also depends on whether the insurance market is genuinely neutral or whether it can be captured or influenced by interests that are not transparent. The mechanisms are technical, but the stakes are not.

This article was produced with AI assistance and reviewed against our editorial standards.