Russia's droneincursioninto Polish territory on September 10 failed to result in much physical damage on the ground, but it did uncover a major hole in Europe's defense forces: The allies don't have cost-effective counter-measures to deal with low-cost Russian attacks.

As many as 19 drones flew into Polish -- and thus NATO -- airspace. Four appeared to have been shot down.

An explanation from some European officials, who spoke to RFE/RL on background, is that priority was given to those drones that seemed to be headed toward critical infrastructure.

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Given the limited damage to property and no casualties on the ground, such a strategy seemed justified. But many are asking why there was a need to rely on expensive AWACS and fighter jets such as F-35s and not a much cheaper anti-drone system, such as Sky CTRL, that Poland uses.

The answer could be, of course, that the air force automatically engages when an interloper flies above a certain altitude. In many countries that is usually anything above 3 kilometers.

NATO chief Mark Rutte said the alliance's response was "very successful" and "showed that we are able to defend every inch of NATO territory, including, of course, its airspace."

Still, the incident poses awkward questions, especially if the Kremlin continues to test NATO countries in a similar way in the future. It also exposes holes in how the alliance has prepared, or not prepared, for a new age of warfare where drones are used extensively.

NATO's response to the drone incursion appears financially unsustainable and militarily inefficient.

As one European official, speaking under condition of anonymity, told RFE/RL: "Air-to-air missiles are extremely costly to use against cheap Russian drones, and if 800 are being sent at once, which happens in Ukraine, Europe will burn through its entire arsenal in just weeks."

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It was quite telling that NATO's North Atlantic Council -- which met the day after the incursion -- requested a review of the military alliance's eastern flank deterrence for potential gaps.

NATO's commander is likely find some glaring ones. Only this year there have been incidents with alleged Russian UAVs (unmanned aerial vehicles) in Bulgaria, Latvia, Lithuania, and Romania. In August, one drone slipped through undetected before exploding on Polish soil, according to the country's military.

European Commission President Ursula von der Leyen's annual state of the European Union speech this week made it clear the bloc needs to up its drone defense when she committed to a 6 billion loan to enter a "drone alliance" with Ukraine.

But more intriguingly she also floated an "Eastern Flank Watch" that she said will provide "real-time space surveillance" to track all aerial movement from the Baltic to the Black Sea.

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She also committed to building "a drone wall" -- something her own commission initially rejected to fund when Estonia, Latvia, and Lithuania asked for European cash earlier this year for just such a venture.

There are few details about all this just yet: no concrete budget figures, precise geographical scope, or timeline other than that the European Commission is expected to present a roadmap with new common defense projects in October.

But the fact remains that the drone wall, the flank watch, and actual drone production are part of the same picture -- and it's one being drawn up with or without EU funding.

Map: Poland, Latvia, Lithuania Restrict Airspace During Russia-Belarus War Games

From Baltic To Black Sea Air Defense

First of all, there are two ongoing and similar projects: the Baltic trio's "Baltic Defense Line" and Poland's "East Shield."

Both projects, announced last year, will take up to a decade to complete and are designed to "fortify" borders with Belarus and Russia, including the Kaliningrad exclave. Billions of euros have already been earmarked.

Plenty of investment will go into classical, physical obstacles such as hundreds of bunkers, fences, ditches, and moats. But there is also the technological aspect to these defense lines.

The Latvian-Russian border near Pasiene, eastern Latvia, pictured in 2019.

The idea is that imagery and signal intelligence and acoustic monitoring will be used for surveillance. Base stations spread out along the defense line, as well as masts, will process the data and connect with weapon systems -- crucially drones designed to take down enemy drones.

This is the drone wall, at least in theory. What first started as an Estonian defense industry idea last year to cover the Baltic trio, dubbed the Baltic Drone Wall, has morphed into a NATO idea -- the Drone Wall -- as German firms are keen to invest and Norway, Finland, and Poland are keen to get onboard as well.

If the EU as an institution comes onboard with funding, it is quite possible this wall will stretch even further south even though Ukraine, arguably the leader in drone technology, essentially is covering countries like Hungary, Slovakia, Bulgaria, and Romania with its drone arsenal.

The key now is what this wall will look like and when it can be installed.

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The most complex models have up to five levels that include acoustic sensors, mobile cameras, radars, jammers, and drone interceptors.

With all these aspects included, it is estimated it may take two to three years to set up as tens of thousands of sensors need to be planted along the borders. Depending on the geographical scope, the main challenge will largely come from sharing and integrating all the information from all systems.

Ideally, NATO should be in charge. But the bigger issue is how and whether it will work in practice.

While most technologies are more or less ready and items such as sensors easily can be scaled up, the issue right now is drones. And it is here Ukraine and the potential "drone alliance" come into the picture.

The country currently produces some 4 million drones a year and hopes to double that capacity. Most of these drones will obviously be needed in the country as it fights off Russian attacks.

What the Europeans instead want from Kyiv is know-how.

A European official with knowledge of the matter told RFE/RL that the idea rather is to use the models and engineering ideas being tested on the Ukrainian battlefield and then scale it up inside the block, potentially with the help of European automotive giants that could free up capacity.