Tripwires Won’t Stop Tanks: NATO’s Forward Defense Problem

European leaders face a narrowing window to rearm and reinforce NATO’s eastern flank before the balance tilts further.

Picture: NATO

As the war in Ukraine drags on, it is becoming increasingly clear that Europe faces two distinct deterrence challenges in the years ahead.

First, unless Russia is convincingly defeated in Ukraine — a prospect that appears unlikely at this stage — European NATO states will likely need to mobilize some form of “deterrent force” capable of deploying into Ukraine to guarantee a ceasefire or peace settlement.

Second, given Russia’s explicit revisionist ambitions and clear signals that it may not stop with Ukraine, NATO must be prepared to deter any direct attack on a member state. Ideally, and in light of the heavy costs associated with even short-term Russian occupation, this means building a credible forward defense posture capable of halting a Russian attack at the border.

These are two substantial challenges for European decisionmakers and societies. Given the current state of Europe’s armed forces, either task alone would be formidable. Taken together, they raise serious questions not only about the political will of European leaders — so far largely underwhelming — but also about their capacity to meet the moment. This is especially concerning as Russian rearmament may already be underway.

Russian mobilization momentum?

Overall, macro-economic indicators continue to look bleak for Russia in the short, medium, and long term. In the short term, high inflation, elevated interest rates, a weak currency, and falling oil prices are straining the economy, affecting both the average citizen and the government.

In the medium term, over-mobilization of labor and resources is constraining further expansion of the war economy while driving up costs due to labor and material shortages.

In the long term, the scale of Russian casualties in Ukraine will weigh heavily on an already fragile demographic outlook. Hundreds of thousands of men who have died will no longer contribute to the labor force. Those returning maimed or disabled are more likely to burden the social security and pension system than support it. Combined with limited access to advanced technology and weak innovation, these factors fundamentally depress Russia’s future economic growth prospects.

That said, Russia’s current societal and industrial mobilization poses a challenge for both Ukraine and Europe. Despite taking heavy casualties, the Russian armed forces appear capable of expanding in size and building a reserve arsenal for potential future contingencies, including against NATO.

Relatively recent intelligence reports suggest that Russia has mobilized enough personnel not only to offset combat losses but also to grow its armed forces. Western officials have similarly noted that Russia is producing more equipment and ammunition — including modern tanks (by Russian standards) and artillery shells — than it is currently sending to the front.

This point is critical. Russian frontline troops often appear poorly equipped. In recent months, footage has surfaced showing assaults across open terrain using civilian cars, buses, mopeds, tractors, and even e-scooters, which are often quickly destroyed by Ukrainian defenders. Reports about Russia’s production capacity indicate that Russia could better equip these forces; it is simply choosing not to. Putin and his circle, with their general disregard for human life and recognition that production capacity poses a more serious bottleneck than manpower mobilization, likely view the current loss of life as acceptable and prioritize stockpiling equipment for future operations and contingencies.

This presents a serious concern for NATO. Western analysts and officials have repeatedly warned that once the war in Ukraine ends and Russia is able to reconstitute its forces, the threat to NATO will grow substantially. If Russia manages to rearm while the war is still ongoing, it effectively shortens the window for European rearmament.

A deterrence force for Europe and Ukraine

Large-scale European rearmament is necessary for two reasons: first, NATO must be able to forward defend its territory in any future war. Second, Europe must be capable of assembling an effective deterrent force to deploy into Ukraine in support of a ceasefire or peace settlement.

NATO’s posture in the East has been based more on forward presence than forward defense. This is especially evident in the Baltics, where each state fields only one to two combat-ready brigades, reinforced by a NATO battalion or brigade-sized battle group. It is unlikely that this national and foreign presence is enough to fully convince Russia that a determined attack would be halted at the initial line of contact.

That’s bad news for two reasons. First, depending on the scenario Russia envisions, an initial breach of the border may be sufficient to establish a fait accompli and aggressively sanctuarize the newly occupied territory — that is, to signal that any attempt by NATO to retake it would trigger nuclear escalation. Second, the war in Ukraine has shown that even brief periods of Russian occupation bring widespread death and destruction.

The informal ‘tripwire followed by a counterattack’ doctrine, which NATO is effectively relying on today, is no longer sufficient. But to establish a credible forward defense posture, European NATO states must significantly increase their presence along the Eastern flank, particularly in the Baltics, which remain the weakest link in the frontline — not due to a lack of commitment from the Baltic states, but because of the inherent manpower and financial constraints they face, as well as the lack of strategic depth.

Estimating what is needed to mount a forward defense is difficult. In general, since defense is considered easier than offense, it is assumed that an attacker needs at least a 3:1 advantage in forces, though this varies with geography, force composition, and other operational factors. Superiority in short- and/or long-range fires, for instance, can shift the required ratio in either direction.

Force deployment is also constrained by what is known as the force-to-space ratio. Simply put, the number of troops that can be concentrated in a given area is limited by available space, shaped primarily by geography and transportation infrastructure. For NATO, this means identifying likely points of attack, estimating the maximum force Russia could realistically bring to bear (both in a surprise scenario and with preparation), and ensuring that NATO’s posture prevents the attacker from achieving a 3:1 advantage, while accounting for offsetting capabilities.

Offering a precise assessment of what is needed to operationalize a forward defense posture is beyond the scope of this short post, but it is pretty clear that current force levels are inadequate. Germany’s planned deployment of a full tank brigade to Lithuania — expected to reach 4,800 fully equipped soldiers by 2027 — is a positive development. Still, even this deployment functions more as a political signal reinforcing the tripwire than as a stand-alone measure capable of forward defending Lithuania (and its neighbors). Several additional brigades will almost certainly be required across the Baltics and Eastern Europe, particularly if the United States reduces its military presence on the continent.

Simultaneously, Europe will face the task of establishing a deterrent force for Ukraine — one that can enforce a ceasefire or peace agreement by deterring renewed Russian aggression. While there is still significant uncertainty about what such a force would have to do and be capable of, it would likely need to consist — at minimum — of 20,000 to 40,000 troops, or roughly six to twelve brigades, capable of conducting high-intensity operations in Ukraine.

In total, this likely means NATO must mobilize some ten to twenty additional brigades — and do so sooner rather than later, especially if Russia maintains or accelerates its current rearmament momentum (subject to the course of the war in Ukraine). This requires an ability to overcome entrenched peacetime procurement cycles, recruitment gaps, and domestic opposition to defense expansion. These aren’t just technical hurdles — they are deeply political, and in many European states, unpopular. Compounding the challenge, European states lack the broader infrastructure needed to support such a buildup of forces.

New and reintroduced technologies can perhaps provide some relief and lower manpower requirements somewhat. Tactical and operational-level drones, combined with anti-personnel and anti-vehicle mines, can significantly contribute to creating an environment of ‘persistent denial’. This being said, they are no silver bullet. It’s also important to keep in mind that Russia not only has more experience in employing these types of systems, they also have significantly more experience in overcoming them.

In the end, Europe’s key problem in rearming is that, while its demographic, economic, and industrial outlook is clearly better than Russia’s over the medium to long term, the short-term prospects currently favor Russia. And if Europe fails to act, it is the short term that may ultimately matter.