The Long War: Fighting Beyond the First Battle

The first of a series of RUSI Military Science outputs in the Long War Programme, this commentary is part of a programme that seeks to examine the cross-domain and cross-government implications – for the UK and its allies – of preparing for and deterring an enduring conflict with Russia.

Wars are rarely short. Despite the hopes of politicians and planners, they often last longer, cost more and claim more lives than anticipated. This has held true from British forces departing for France in 1914, to NATO’s foray into Afghanistan, to Vladimir Putin’s ongoing war against Ukraine. Put simply: very rarely is a war over by Christmas.

As NATO reorientates to face a post-Ukraine Russia, the United Kingdom must resist the institutional impulse to plan for the first battle alone. The next war may not be quick and winning it will demand more than readiness; it will require breadth and depth as a priority to build resilience and political honesty about the realities of prolonged industrial-scale conflict.

First Battle Bias and the Fragility of Readiness

The UK’s defence planning remains shaped by what might be called a ‘first battle bias’: the natural tendency of institutions to prepare for the most immediate, visible challenge. Forces that must demonstrate readiness are under pressure to prioritise capabilities that are rapidly deployable, easily exercised and politically expedient. This creates an incentive to invest in exquisite platforms and headline capabilities; and a corresponding tendency to neglect less demonstrable forms of resilience such as reserve forces, deep stockpiles and industrial mobilisation. This is evident from the United Kingdom’s poor force ratios of regular to reserve compared to our allies and our allies investments in rapidly growing their reserve forces.

A New Framework, an Old Problem

At RUSI’s 2025 Land Warfare Conference, General Sir Roly Walker introduced a new framing for the British Army as a ‘20-40-40’ force. Under this model, 20% of the force is intended to be survivable: tanks, air defences, critical C2 systems; and - most importantly - people. A further 40% is “attritable”: expendable equipment we would prefer to keep but can afford to lose. The final 40% is consumable; weapons like FPV drones, loitering munitions and other single use assets that reflect the ‘cheap mass’ tactics emerging from Ukraine.

This rebalancing is a welcome step. But it leaves an awkward question unresolved: what happens when this survivable 20% is damaged, destroyed or killed?

Despite clear recognition of the requirement for improving future combat power, there remains little evidence that the UK has a plan to fight a war lasting more than a few weeks. Medical capacity is limited. Reserve regeneration pipelines are slow. There are no clear arrangements for reconstituting high-value capabilities or training sufficient replacements. The British plan for mass casualty outcomes appears to be based on not taking casualties. This could be considered an optimistic planning assumption.

A long war requires more than just a first echelon of high-readiness forces. It demands a second and even third echelon; personnel, platforms and logistics chains that can absorb losses and continue the fight. Yet this depth is notably absent from current British force design.

Reserve forces, the natural source of this second echelon, remain under-resourced and under-equipped. According to DSTL, reserves offer a significantly cheaper way to generate military mass, often five times more cost-effective than regular forces. But because their readiness is harder to demonstrate and their outputs less immediately visible, they are often side-lined in favour of regular capabilities that offer clearer short-term return.

The result is a brittle force. The SDR’s promise to grow the number of active reservists ‘by 20% when funding allows, most likely in the 2030s’ is an illusory commitment. It lacks urgency, accountability and coherence. Given the likely trajectory of defence budgets, it is doubtful any future government will be held to such a vague promise, especially when regular force shortfalls are always closer to hand and seemingly more politically embarrassing. This was evident when General Sanders made the fairly innocuous point that in the event of war, a citizen army would be required but this was mired by scandal and a largely irrelevant debate around conscription.

An Immoral Gamble

It is not only impractical to plan for reconstitution to be ad hoc and ‘on-the-night’, it is an immoral gamble. A government that intends to mobilise reserves or raise a citizen force has a duty to equip and train them properly. Failure to do so risks sending underprepared soldiers into lethal environments without the means to survive or succeed.

As RUSI analysis of Ukraine’s 2023 offensive has shown, collective training limitations were a major constraint on combat power generation and performance on the battlefield. The same applies here. Without platforms and vehicles on which to train, reserve or citizen forces cannot reach even a baseline level of combat effectiveness. The focus on individual reinforcements from volunteer reserves to plug regular force gaps has seen training for volunteer reserve unit command teams wither away. Mobilisation of a second and third echelon cannot happen without a clear plan to arm them with the complex weapons required for modern warfare; and crucially, this must happen in sequence to provide substantial collective training using those platforms. Training using emulation and simulation offer a cost effective but underutilised and underfunded methodology to improve reserves competency where time is a premium.

Mobilising a second echelon without equipping them would expose catastrophic shortfalls in tanks, artillery, enablers and collective training infrastructure. It would constitute a failure of moral and legal duty inviting political scandal.

This is not theoretical. During the Iraq and Afghanistan campaigns, poorly-equipped reservists were sent into combat in vulnerable Snatch Land Rovers, resulting in preventable deaths. In a future war with Russia, the consequences would be far greater, with the possibility of thousands of preventable casualties as a result of peacetime inaction.

Industrial Defence in Depth

Equipping a second echelon is not simply a matter of duplicating the regular force structure. Novel solutions such as dissimilar rearmament, adopting cheaper, simpler platforms and weapons designed to be mass-produced, must be seriously explored. These may not win in a peer-to-peer contest on day one, but the second battle will be a different form of conflict to the first.

Britain must also address the broader question of defence in depth. This includes stockpiles, industrial surge capacity, casualty replacement and renewed legal frameworks for mobilisation. The Defence Readiness Bill is an opportunity for this Government to contribute to the solution of this problem. But as it stands, the UK has not yet demonstrated a credible plan to regenerate and increase the scale of combat power beyond the first weeks of a war.

Endurance is Deterrence

A doctrine of endurance must now replace the illusion of decisive battle. Deterrence does not come only from exquisite readiness or technological overmatch. It comes from convincing a potential adversary that, even if they land the first blow, we can absorb, reconstitute and continue to fight. Russia does not need to outgun NATO; it only needs to outlast it.

To deny them that pathway to victory, the UK must urgently invest in its ability to regenerate force, sustain operations and endure the long war. This means funding reserve transformation, building a second echelon and facing up to the politically uncomfortable truth that war readiness is more than a day-one force.

Without this, Britain risks repeating the mistake that so often characterises modern warfare, preparing for the first battle, but not the next.

© RUSI, 2025.

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