Russian and Belarusian tanks, jets, artillery, and infantry have begun surging toward EU borders in a massive war game that is taking place amid real Russian aggression in Ukraine and Syria.
Called Zapad 2017, the two-week drill is meant to repel an attack by a fictional state, Veishnoriya, and two fictional allies, Lubeniya and Vesbasriya, that look a bit like Nato and EU countries Estonia, Latvia, Lithuania, and Poland, which currently host around 4,000 Nato troops.
According to Belarus and Russia, Zapad 2017 involves 12,700 soldiers and 250 tanks, as well as 200 pieces of artillery, 70 planes and helicopters, and 10 warships on the borders of the Baltic states, Finland, and Poland.
That would put the drill below levels that obliged Russia to invite foreign military observers in large numbers under a treaty called the Vienna Document, with Nato invited to send just three monitors and with a handful of military attaches from Ukraine, Poland, Lithuania, Latvia, Estonia, Sweden, and Norway to also drop by on brief visits.
But the fact that Russia openly requisitioned 4,200 railway wagons for the manoeuvres has led Germany and Poland to estimate that the real number of troops will top 100,000.
Speaking to Deutsche Welle, a German broadcaster, on Wednesday (13 September), Russian deputy defence minister Alexander Fomin said: "I can calm our dear neighbours straightaway. The exercise is absolutely peaceful, and absolutely defensive in nature".
But for Danish defence minister Claus Hjort Frederiksen, speaking on Danish radio on Thursday, the drill was designed to "intimidate the Baltic states".
There is little concern that Russia might use it to launch a real invasion, but Lithuania's defence minister Raimundas Karoblis, also voiced concern.
"We can't be totally calm. There is a large foreign army massed next to Lithuanian territory," he told the Reuters news agency on Wednesday.
Jens Stoltenberg, the Nato secretary general, warned earlier that Russia's lack of transparency "increases the risk of misunderstanding, miscalculations, accidents, and incidents that can become dangerous".
He said Zapad 2017 was part of a "pattern of a more assertive Russia" that has shown it was "willing to use military force against its neighbours", referring to Russia's invasion of Georgia in 2008 and its covert invasion of Ukraine three years ago.
The US has assumed command of airspace over the Baltic countries as a precaution and Ukraine has put its armed forces on alert.
For his part, the Polish deputy defence minister, Michal Dworczyk, said that even if the war game was peaceful, it could result in an increased threat in future.
"There are many doubts and fears that, because of the Russian Federation's previous actions, not all forces and equipment will be removed [from Belarus] after the exercise," he told reporters in Warsaw on Wednesday.
Phillip Petersen, a US defence analyst at the Potomac Foundation, a think tank in Washington, told Polish news agency Pap that Zapad 2017 looked like "preparations for an invasion of the Baltic states and Poland".
"We are heading into a dangerous period. It's not just about the two-week exercise, but probably also about the next two months, because the exercise helps them to be ready for an attack", he said.
The Russian drill comes at the same time as a Swedish exercise involving 19,000 mostly Swedish troops and 1,500 soldiers from the US, France, Norway, and other Nato members.
Sweden, which is not part of Nato, has said its exercise, called Aurora, was not timed to challenge the Russian show of force.
Nato soldiers will take part in a large Polish drill, called Dragon-17, on 25 September.
The Western alliance will also hold six small exercises in the Mediterranean Sea and in Germany, Bulgaria, Slovenia, Romania, and Turkey this month.