U.S. and British troops carried out a large-scale North Atlantic Treaty Organization defensive drill on the border between Poland and Lithuania – called the Suwalki Gap.
The coalition troops, for the first time, rehearsed for a possible combat scenario in which Russia might attempt to annex the Baltic states from Western alliance.
U.S. helicopters and British aircraft took part in simulated defence exercises with their Polish, Lithuanian and Croatian counterparts.
"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," U.S. Lieutenant General Ben Hodges told Reuters.
If the Gap is seized by Russia, it would cut off Estonia, Latvia and Lithuania.
But, on Monday, NATO Secretary General Jens Stoltenberg stated that there are no plans to increase troop numbers in Baltics and Poland. "We do not plan to increase the number of troops as part of a battlegroup, we think we will have a measurable defensive, but also sufficient, presence through the battlegroup," he said at a briefing.
Stoltenberg said the battlegroups would be enhanced occasionally through military exercises. After which, he underscored "we do not see any imminent threat against any NATO ally, including Latvia."
Russia has denied harboring any plans to invade the Baltics and accused NATO of threatening the stability in Eastern Europe, by its military presence and staging of combat practice.
Hodges said it was crucial for the alliance to display readiness. "We have to practice, we have to demonstrate that we can support allies in keeping (the Gap) open, in maintaining that connection," he said.
Russia's 2014 seizure of Ukraine's Crimea peninsula altered NATO's perception into viewing Russia as a potential adversary.
Prior to that incident, there were no alliance members forces stationed in the Baltic states. Now, there are a combined 4,500 troops from Estonia, Latvia, Lithuania and Poland.
While 1,500 troops took part in the exercises, a Lithuanian commander cautioned that it would take more to defend the Gap in the event of a genuine conflict. "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.
"The training helps present a credible defence force that hopefully will deter aggression, but if not, we'll be prepared to move to defend the borders of NATO," said Lt. Col. Steven Gventer, who leads the U.S. battlegroup in Orzysz.
NATO officials speculate that Moscow will hold its own exercise in Russia and Belarus on a much greater scale in September – possibly with 100,000 troops.
Baltic officials believe Moscow will also rehearse an attack on the Suwalki Gap.
"I think it's important for the soldiers to train on land that they may have to defend some day," said Major General John Gronski, deputy commander, U.S. Army Europe, observing the exercise in Lithuania.