In the driving rain, the men from 45 Commando jumped from their Chinooks and made for the cover of a small wood on the Polish-Lithuanian border.
Backed by US and Polish troops, the Royal Marines had landed in the Suwalki Gap for the first large-scale Nato defensive drill in the area, amid fears of European vulnerability to a Russian assault.
A slender 60-mile wide stretch of undulating rural land sandwiched between the Russian enclave of Kaliningrad and Moscow’s ally, Belarus, the Gap is regarded as Nato’s weak spot on its eastern flank.
If war were to break out between east and west, this is where Russia could make its first move to cut off the Baltic states from Poland in one swift move.
Since Russia seized and occupied Ukrainian Crimea in 2014, eastern Europe has felt increasingly vulnerable, and nowhere more so than at the Gap.
The threat has prompted Nato to deploy some 4,500 troops to Lithuania, Poland, Latvia and Estonia, and, after prompting by the Poles, conduct a series of US-led war games, involving some 1,500 troops, called Sabre Strike, to help prime forces for potential combat.
The enemies of Sabre Strike went by the fictional name Bothnians, and were played by the Lithuanian Army, but nobody was under any doubt who they really represented. Ask Lithuanians and they nod to the east, in the direction of Russia, their giant neighbour that casts an ominous shadow over the comparitively tiny Baltic state.
“The Gap is vulnerable because of the geography. It’s not inevitable that there’s going to be an attack, of course, but ... if that was closed, then you have three allies that are north that are potentially isolated from the rest of the alliance,” US Lieutenant General Ben Hodges said.
Moscow is expected to hold its own, much bigger exercise in the region in September, during which Nato officials believe 100,000 troops will rehearse an attack on the Suwalki Gap.
“This is only a small-scale drill compared to what would be needed in case of a real attack, but it is important for us because it shows that allies share our worries”, said Brigadier General Valdemaras Rupsys, head of Lithuania’s land forces.
Sitting in the ramshackle and dilapidated old Lithuanian army base that served as a camp for the British troops, Major Simon Cox explained that his men – along with further contingents from the US and Poland – had to regain control of a key road linking Lithuania to Poland that an attacking “enemy” had seized some 12 days before.
Once in control of the road they were to hold it until a convoy of British and US vehicles arrived.
To take the road required the Marines to launch a night-time assault through the patchwork of mosquito-infested marshes, woods and farmland that comprise the countryside in the Suwalki Gap, and attack a force Major Cox said was determined to hone its defensive skills.
“The Lithuanians are not there to support our training: they are there to support their own training,” he told The Daily Telegraph the day before the exercise started. “They have no interest in making our job easy or giving us an easy ride.”