There are a number of potential vulnerabilities in NATO territory if the Cold War ever goes live, but three countries are especially scared of an attack to close the Suwalki Gap, a spot where NATO territory is only 60 miles wide with Belarus on one side, Russia's Kaliningrad Enclave on the other, and relatively flat ground with little forest or natural defenses in the middle.
If Russian tanks and other forces quickly span the gap and dig in, they could cut Lithuania, Latvia, and Estonia off from the rest of Europe and NATO for easy digestion, leaving the rest of NATO to decide whether its worth it to risk their own troops re-securing them.
These sorts of geographic and political choke points aren't new. Cold War weapons development was heavily influenced by the Fulda Gap, one of Soviet Russia's most likely routes of advance if they invaded western Europe. And in World War II, France fell so quickly partially because it had counted on holding Germany at the Albert Canal and River Meuse in Belgium, but Germany had gotten Belgium to rescind its alliances with France before the invasion.
But the Suwalki Gap is a particularly vulnerable and important point. Kaliningrad is a Russian enclave that borders the Baltic Sea, and Belarus is a historic ally of Russia. Russia could easily muster troops in both areas for a mad dash across the line, and it could do so quickly and with little warning.
On paper, NATO would be required to respond, but NATO's joint defense clause has only been invoked one time, and that was when America was attacked on September 11, 2001. That's part of why deploying to Afghanistan usually results in a service member receiving a NATO medal. The whole alliance was party to the invasion because one member of the alliance had been attacked.
But that was NATO allies backing up their most powerful member while invading a relatively weak, authoritarian state. The Taliban had little air force or proper army assets, and it quickly fell. Getting NATO allies to muster in Poland for an attack into Lithuania, Latvia, and Estonia would be much more challenging, especially since many countries would be interested in holding back at least some of their military forces to defend their own borders as the situation developed.
Poland might be especially reluctant. The gap shares its name with a Polish city and a Polish County, and Poland shares a much larger border with Kaliningrad and with Belarus. It might legitimately need its troops to hold the line against further Russian attacks.
Meanwhile, remember how Russia is under a lot of pressure to get back in line with that INF Treaty that bans missiles of certain ranges, especially ones that can carry nuclear warheads? Well, a lot of the Iskander and Iskander-M systems that are likely in violation of that treaty are kept in Kaliningrad, where they could threaten U.S. and NATO ships in the Baltic Sea and army formations approaching Suwalki from the south.
An Iskander launcher could likely cover the entire Suwalki Gap from any point in Kaliningrad. Multiple launchers could take turns shooting and then scooting before they could be hunted down. Meanwhile, Russia's invasion force would be north of the gap, conquering the NATO countries and setting up defenses ahead of the NATO counterattack.
The Suwalki Gap is something Lt. Gen. Ben Hodges talked about when he was the commanding general of NATO, saying that an invasion of NATO countries wouldn't make any sense to him, but then neither had the seizure of Crimea or the Russian deployment to Syria.
For comparison, while the Suwalki Gap is 60 miles long, the annexation of Crimea created an approximately 85-mile-long front between Russia and Ukraine, though NATO wasn't obligated to take part in that fight and much of that border is covered in water, making a Ukrainian counterattack much more risky and challenging.
Of course, all of this only matters if Putin is ready to do something even crazier than kidnapping an Estonian intelligence officer, annexing Crimea, or invading Georgia or the Donbas. While all of those actions were baffling for international observers, the annexed territory, at least, did have populations sympathetic to Russian rule.
While Estonia, Latvia, and Lithuania do have some Russian-speaking populations, the countries as a whole strongly support NATO membership and have even inked agreements to move more U.S. and other NATO troops into the countries or allow more forces to transit through them. So, Russia would likely find it challenging to actually hold the countries permanently.