FORTS ARE back in fashion. Ukraine’s counter-offensive last year was stymied by the so-called Surovikin line: a sprawling array of Russian minefields, trenches, anti-tank obstacles and old-fashioned barbed wire, among other impediments. As Ukrainian forces slowed down to clear mines, bridge ditches and bulldoze obstacles, they were observed by drones and hit with a hail of anti-tank missiles and suicide drones. So uncharted was this territory that Valery Zaluzhny, then Ukraine’s top general, asked his staff to dig out “Breaching Fortified Defence Lines”, a book by a Soviet major-general. It was published in 1941.

NATO armies have been taking notes. In January the defence ministers of Estonia, Latvia and Lithuania announced that they would build a string of “anti-mobility defensive installations” along their border with Russia and Belarus, collectively known as the Baltic Defence Line. “Fortification measures have played a significant role in wars in our region in history,” says Susan Lillevali, an Estonian defence official, pointing to the example of the Soviet-Finnish war. “We have also studied the Russian war in Ukraine,” adds Lieutenant-Colonel Kaido Tiitus, a commander in the Estonian Defence League, a volunteer organisation. “Our main lesson is that we need to find a way to stop the advance of Russian armoured units.” The Baltic states will not have been assured by the promise, made by Vladimir Putin in an interview on February 8th with the former Fox News journalist Tucker Carlson, that he had no plans to invade the Baltics, Poland or anywhere else beyond Ukraine.

Estonian officials estimate their stretch of the border will need around 600 concrete bunkers, each 35 square metres, each capable of holding around ten soldiers and taking a hit from a large shell. Prototype bunkers are being developed and construction is expected to start next year, at a cost of around €60m ($65m). The aim is not to create an impregnable fortress but to slow invaders, wear them down and buy time to bring up reinforcements. If Latvia and Lithuania were to build bunkers at a similar density, they would need 1,116 bunkers and 2,758 respectively, calculates Lukas Milevski, an expert at Leiden University in the Netherlands.

The catch is not military engineering but democratic consent. “The most important part is agreement with landowners,” says Ms Lillevali, noting that most of the borderlands are privately owned. She says there has been little sign of pushback from Russian-speaking minorities in the area. Locals may be reassured by the fact that the armed forces do not plan to store explosives near the strongholds in peacetime, nor install anti-personnel landmines, which are illegal under the Ottawa Treaty. An effort in Estonia’s parliament last year to withdraw from that treaty made little progress.

The appeal of fortifications is easy to see. European officials worry that Russia’s breakneck rearmament is outpacing Europe’s own effort to ramp up arms production. Baltic leaders have emphasised that even small Russian advances could be existentially threatening to their states. “It cannot be ruled out that within a three to five year period, Russia will test Article 5 and NATO’s solidarity,” warned Troels Lund Poulsen, Denmark’s defence minister, on February 9th. “That was not Nato’s assessment in 2023. This is new information that is coming to the fore now.” In light of this febrile mood, the Baltic Defence Line is both a military statement and a political one.

But Russia’s successful defence has also prompted a wider rethink. Russian fortifications in southern and eastern Ukraine were the most extensive defensive works in Europe since the second world war, according to analysis by the Centre for Strategic and International Studies, an American think-tank. They are probably rivalled only by the minefields and obstacles on the inter-Korean border. In November Volodymr Zelensky, Ukraine’s president, urged his commanders to accelerate the construction of defences in the east. Poland, too, is building up fortifications and shelters along its border with Russia and Belarus, an ally of the Kremlin.

This throws up a dilemma. NATO armies have long preferred a more elastic defence in depth, in which forces retreat as needed and destroy the enemy on more favourable terrain. That is incompatible with defending every inch of NATO soil. But with an “operationally static defence”, observes Mr Milevski, “it’s much more of an imperative to ensure that the blow, when it comes, is as weak as possible”. That puts a much greater emphasis on using heavy firepower to strike deep behind Russian lines to wear down the attacking force and break up its command and logistics. In short: heavy bombardment of Russian soil. “Western political leaders,” he warns, “may be squeamish about such attacks.” ■