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.