SEPTEMBER will be an edgy time for NATO’s front-line member states. For a week in the middle of the month, Russia will be running what is being described as the biggest military exercise in Europe since the end of the cold war. The build-up is already under way.

Zapad (“West”) exercises take place every four years and date from Soviet times, when they were used to test new weapons and tactics. Zapad 2017 is expected to involve at least 100,000 Russian troops. It will extend across the country’s Western Military District and Belarus, which has a border with three NATO members. By next week, most of the advance elements of the forces taking part in the exercise will have arrived. The rest, expected a fortnight later, will include the First Guards Tank Army, a famous unit from the second world war that was reformed in 2015. It packs a mighty offensive punch.

Previous Zapads alarmed NATO because of their size and because of the kind of war game they have played out—one in 2009 included a simulated nuclear attack on Warsaw. But this year’s is the first to be held since Russia’s aggression against Ukraine. It unfolds against the backdrop of a relationship with the West more tense and adversarial than at any time in 30 years. Earlier this year, as part of its response to Russia’s annexation of Crimea and covert invasion of eastern Ukraine, NATO deployed four battalion-sized battle-groups to Poland, Estonia, Latvia and Lithuania. Essentially a tripwire consisting of only about 4,000 troops, these multinational units are meant to send a message to Moscow that if it tries anything against a NATO member in the east, it will quickly face the whole alliance. Last month an American-led NATO exercise called Sabre Guardian saw 25,000 troops from more than 20 countries carrying out drills across Hungary, Romania and Bulgaria. The idea was to practise territorial defence against a technologically sophisticated aggressor. Yet there are big differences between the way NATO conducts military exercises and the way Russia does. Zapad apart, since 2013 Russia’s president, Vladimir Putin, has ordered a series of no-notice “snap” exercises, often involving up to 50,000 troops and anti-Western scenarios. There is little doubt that Mr Putin sees these not just as a means of honing efficiency, but as a way to intimidate smaller neighbours and eventually draw them into a sphere of Russian influence. Four such drills took place in 2013. A fifth, which began in late February 2014 and deployed large numbers of airborne troops, armoured vehicles and attack helicopters, became the springboard for the occupation of Crimea. Even before he turned to snap drills, Mr Putin used an earlier exercise, Kavkaz 2008, to launch his invasion of Georgia. Military deception, or maskirovka, is an art at which he excels.

General Ben Hodges, the commander of American forces in Europe, frets that Russia may be planning to use Zapad 2017 to bring soldiers and kit into Belarus and leave them there. Last month he said: “People are worried this is a Trojan horse. They [the Russians] say, ‘We’re just doing an exercise,’ and then all of a sudden they’ve moved all these people and capabilities somewhere.” In particular, NATO is worried about the movement of longer-range missiles, surveillance drones and special forces into threatening positions under the pretext of the exercise.

There are even fears, based on intelligence sources, that Zapad 2017 could be used to insert forces into Belarus that would shore up Russian influence in the event that its dictator, Alexander Lukashenko, either moves closer to the West or is deposed by someone careless of Moscow’s interests. Russia’s deputy foreign minister, Grigory Karasin, responded to General Hodge’s remarks as might have been expected. “This artificial buffoonery over the routine Zapad 2017 exercises”, he said, “is aimed at justifying the sharp intensification of the NATO bloc [activities] along the perimeter of Russian territory.”

Russia has brought suspicion on itself. It routinely flouts the Vienna Document, an accord designed to avert misunderstandings during war games. Brokered through the Organisation for Security and Co-operation in Europe, it fosters transparency. Any drill involving more than 9,000 troops requires advance notification of at least 42 days; any exercise involving more than 13,000 troops must be preceded by an invitation to the other 56 participating states to send two observers.

Russia breaks both rules with its snap exercises, and the second with Zapad. As a cynical way of dodging its obligations, it often claims to be holding a series of different drills at slightly different times. At the last meeting, in July, of the NATO-Russia Council, a body set up 15 years ago to improve co-operation and understanding between the alliance and Moscow, both sides did at least give briefings on planned exercises.

However, NATO’s secretary-general, Jens Stoltenberg, a former prime minister of Norway, was doubtful about the reported numbers for Zapad and urged Russia to obey the Vienna rules. His scepticism is well-founded. The Russians said before the last Zapad that it would involve just 12,000 troops. In fact, more than six times as many took part. Estonia’s defence minister, Margus Tsahkna, recently revealed that Russia has requisitioned 4,000 railway carriages to take troops and equipment to Belarus.

Especially galling for NATO is the way Russia uses the Vienna Document to ensure it has its full complement of observers at its perceived enemy’s big exercises. All NATO can do is remain vigilant and hope Mr Putin sends his troops back to barracks when Zapad is over. General Hodges says: “Look, we’ll be ready; we’ll be prepared. But we’re not going to be up on the parapets waiting for something to happen.”