Can US allies build enough independent defense capacity to maintain deterrence if Washington reduces or conditions its military commitments?

Advocate A’s weakest point is the move from “the shopping list is identifiable” to “therefore allies can fill it if repricing is phased.” Identifying a capability gap is not evidence that it can be closed on the timeline relevant to deterrence. Deterrence is forward-looking but immediate: Russia, China, or North Korea would not wait until 2030 or 2035 for European and Japanese procurement cycles to mature. A is right that burden sharing should mean usable forces rather than headline spending, but that concession actually supports Advocate B: higher budgets are necessary, not sufficient, and the transition gap is the strategic danger.

The first counter-evidence is that the U.S. role under review is not merely symbolic troop presence. AP reported on June 18, 2026 that Hegseth announced a six-month review of U.S. forces in Europe, and that the U.S. had already signaled on June 3 that it would no longer supply an aircraft carrier and support ships, aerial refueling planes, and dozens of fighter jets in a crisis. Those are classic enabling assets. If they are withheld, Europe is not just replacing billets in Germany or Poland; it is replacing theater logistics, airpower surge capacity, maritime strike, refueling, and command integration.

Second, NATO’s own timelines undercut A’s confidence. The June 2025 capability targets and 5% GDP framework are explicitly designed to strengthen warfighting readiness “for years to come,” with the spending benchmark running to 2035. That is a decade-long capacity plan, not an immediate substitute for a diminished U.S. guarantee. A 2035 target may improve long-run burden sharing, but it does not preserve deterrence during 2026-2030 if Washington conditions crisis support.

Third, the hardest gaps are not easily bought. IISS identifies U.S.-provided theater ISR, space-based intelligence collection and fusion, deep precision strike, integrated air and missile defense, strategic lift, and the nuclear backstop as disproportionate U.S. contributions. In a separate 2026 paper on European military space capacity, IISS judged that even major announced European investments would not let Europe replicate global-scale persistent U.S. ISR, sovereign missile early warning, or U.S.-level space situational awareness within a decade. That directly rebuts A’s “defined shopping list” framing: some items are ecosystem capabilities, not procurement line items.

Fourth, A’s ammunition example is too narrow. CSIS assessed that Ukraine could not fully replace U.S. support in air defense and intelligence assistance for precision targeting, and that Europe would struggle to provide large munition volumes given Ukraine’s daily expenditure and Europe’s weak defense industrial base. Even if 155mm shell capacity expands, deterrence also depends on air-defense interceptors, long-range fires, sensors, targeting, sustainment, and stockpiles.

Japan reinforces the same point. Its FY2023-FY2027 buildup is serious, including standoff defense, integrated air and missile defense, unmanned systems, C2/intelligence, mobility, and sustainability. But the program itself says Japan would defeat invasion while gaining support from its ally and others. That is not independent deterrence; it is stronger self-defense nested inside the U.S. alliance framework.

A’s rebuttal concedes the decisive issue while trying to narrow the test. A admits that if Washington withholds crisis enablers before allies field substitutes, deterrence could weaken immediately; that Europe cannot replicate global-scale U.S. space and ISR architecture within a decade; and that Japan’s buildup remains alliance-based rather than independent. Those concessions imply that A’s policy only works if the United States remains the operational backstop during the entire transition. But the premise being debated is whether allies can maintain deterrence if Washington materially reduces or conditions commitments. If the answer depends on Washington not actually reducing the most important enablers until allies catch up, then A has not shown independent allied capacity; A has restated B’s warning in conditional language.

The weakness is A’s substitution of a “denial posture” standard for the actual deterrence problem. A argues Europe need not duplicate the full U.S. military, only deny a rapid fait accompli. But denial still requires exactly the capabilities now at risk: integrated air and missile defense, operational command-and-control, ISR fusion, strategic lift, aerial refueling, long-range fires, secure space services, and stockpiles. AP’s June 18, 2026 reporting states that NATO’s American supreme allied commander is already making backup plans after the U.S. signaled it would not supply assets including an aircraft carrier and support ships, aerial refueling planes, and dozens of fighters in a crisis; AP also describes this as the United States scaling back how it might help if Article 5 is triggered. (apnews.com) That is not a theoretical future bargaining chip; it is precisely the removal of crisis capacity that makes denial credible.

A’s proposed forcing mechanism also assumes the institutional bottlenecks are easier to overcome than the evidence shows. The European Defence Agency says a recurrent obstacle has been defense ministries planning and procuring in isolation, and the 2024 CARD process lists urgent collaborative opportunities in integrated air and missile defense, air transport, space services, strategic lift, command-and-information systems, satellite observation, and satellite communications. (eda.europa.eu) That list supports B, not A: the urgent priorities are still gaps, not operational substitutes. IISS similarly identifies U.S.-provided enablers as disproportionate and separately assesses that Europe cannot replicate U.S.-level persistent ISR, missile warning, or space situational awareness within a decade. (iiss.org)

Ukraine is the empirical stress test. CSIS concluded that Europe and Ukraine cannot fully replace U.S. capabilities, especially air defense and intelligence assistance for precision targeting, and warned that any replacement must occur before Russia exploits a dip in fighting power. (csis.org) The same transition-gap logic applies to NATO and Japan: budgets and plans improve future resilience, but deterrence can fail during the interval when adversaries see U.S. commitment falling faster than allied readiness rises.

A’s strongest case is that “independent” should not mean autarkic duplication of the entire U.S. military. If Washington reprices commitments gradually and ties them to measurable capability milestones, Europe could build a denial posture against Russia while Japan builds more standoff strike, missile-defense, C2/intelligence, mobility, and sustainment capacity, leaving the United States as a backstop rather than the backbone. A is also right that the capability gaps are now identifiable: ISR fusion, IAMD, long-range fires, logistics, munitions, lift, and command-and-control are concrete targets for pooled procurement rather than abstract complaints about free-riding.

B is still stronger because A’s argument depends on the United States remaining sufficiently committed during the transition to prevent the very gap being debated. The current evidence suggests the gap is not theoretical or far off. Associated Press reporting says the Pentagon review is occurring alongside U.S. signals that, in a crisis, it would no longer provide an aircraft carrier and support ships, aerial refueling aircraft, and dozens of fighters; AP also reports NATO’s American supreme allied commander is already preparing backup plans. Those are not marginal subsidies. They are the operational connective tissue that lets NATO defense plans convert national inventories into timely combat power.

A’s denial-posture standard also understates how dependent denial is on high-end enablers. IISS’s “European Way of War” analysis says Europe could try to deny a rapid Russian success, but it also says that until Europe develops its own integrative architecture, its way of war will be constrained by what it can actually do. IISS’s separate space-capacity paper is more direct: even with major announced investments, Europe could not replicate U.S.-scale persistent ISR, sovereign missile warning, or space situational awareness within a decade. That matters because modern denial is not just brigades and artillery; it is detection, targeting, air defense, communications, lift, refueling, and replenishment under attack.

The procurement record reinforces B. The European Defence Agency’s 2024 CARD process identifies fragmentation as a recurrent obstacle and lists urgent cooperative gaps in IAMD, tactical and strategic airlift, space services, satellite communications, satellite observation, and command-and-information systems. A says conditional U.S. pressure could force coordination; perhaps, but that is a policy hope, not evidence of combat-ready capacity now. Ukraine is the practical warning: CSIS found that Europe and Ukraine could not fully replace U.S. support, especially air defense and intelligence assistance for precision targeting, and that Russia could exploit any dip in fighting power. The same transition-risk logic applies to NATO and Japan. Stronger allied budgets improve future resilience, but deterrence can fail before procurement, integration, stockpiles, and readiness catch up.