"World War III Warning!!" Suwalki Gap Keeps Top U.S. General in Europe, suspicious Russian forces border activity.

RUKLA, Lithuania — A 60-mile long sliver of flat land just inside NATO member Poland gives the U.S Army's commander in Europe sleepless nights.

Lt. Gen. Ben Hodges, the commander of U.S. Army Europe

A map showing the location of the Suwalki Gap. Russia's detached Kaliningrad province — which is about 200 miles from the country's border — can be seen just north of Poland and southwest of Lithuania. Google Maps

It's called the "Suwalki Gap" and should Vladimir Putin decide to invade, it would be perfect for advancing Russian tanks. Sandwiched between Moscow-ally Belarus to the east and Russia's far western enclave of Kaliningrad, it's also why the military alliance is training harder — and faster.

If the Russian president gave the order to sweep across Suwalki, his forces would initially split the Baltics from the rest of NATO before the West could do anything to stop it.

While ISIS in Iraq and Syria may preoccupy his peers, Lt. Gen Ben Hodges worries about the growing number of "snap" Russian war games on NATO's border — live-fire exercises with units at fighting levels, unannounced and unmonitored by Western observers. Several have been held right next to the tiny Baltic states of Lithuania, Latvia and Estonia — NATO'S soft underbelly.

Gesturing from his Black Hawk helicopter while flying over the potential flashpoint, Hodges explained the nightmare scenario. "You get thousands of Russian troops in exercises on both ends of the Suwalki Gap, and now everybody's in the field," he said. "They have equipment — so there's a potential for them to transition from an exercise to an operation — that's our concern."

A view of the Suwalki Gap region taken from a helicopter.

Since he took command in 2014, Hodges has counted dozens of violations of NATO airspace by Russian fighter jets and bombers, often flying with their transponders off, making them difficult to track.

Earlier this month, NATO amassed more than 30,000 troops and hundreds of aircraft and warships in the largest display of Western military strength and speed since 9/11. The exercise included a 5,000-strong rapid-reaction force.

It also offered a message to the three Russian observers — invited by NATO to witness the exercise — to take back to the Kremlin.

"We can mobilize brigades and divisions within days. So this should be quite clear to any potential adversary that NATO is the strongest military alliance that exists today," said Lt. Gen. John Nicholson, NATO's allied land commander and one of its key strategists. "That ability to respond, we think, will actually prevent conflict … we think it will give them pause before they choose to do anything rash or [make] a mistake or miscalculation."

In a sign that the alliance isn't backing down, on Wednesday it invited tiny Montenegro to join — in defiance of Russian warnings that such a move would be a provocation.

Still, if deterrence doesn't stop Putin, NATO commanders agree that any counterattack to take back the Baltics would be extremely bloody. Russian military vehicles have already moved scores of Iskander missiles into Kaliningrad — each nuclear-capable and with a range of 250 miles.

"We are committed to the sovereignty of Lithuania, the sovereignty of Poland and all the other countries, so we will do whatever it takes to reestablish that," Hodges said. "[But] it's not inevitable that it goes to a Third World War. Nobody wants that, including the Russians."

Meanwhile, Polish soldiers man watchtowers around-the-clock along the Suwalki Gap, looking for suspicious Russian border activity. It's an echo back to the Cold War days when some 300,000 U.S. troops waited for a Soviet tank invasion across the Fulda Gap in central Germany.

Today, Hodges has about one-tenth the number of U.S. soldiers to carry out the same mission: deterring a Russian attack on a NATO state.