Putin's invasion in Crimea and his apparent willingness to redraw international borders to have set Eastern Europe and the Baltic States on edge.

Russian forces poured into the strategic Black Sea region under the pretext of protecting ethnic Russians from "lawlessness," setting a dangerous precedent for the Kremlin's intervention in the region.

Nadia Diuk, the vice president at the National Endowment for Democracy, explained on PBS Newshour why Poland is particularly concerned right now:

"There’s a place called Kaliningrad, which is kind of in between Lithuania and — and Poland right now, and that is actually part of Russia that could be a staging ground for all sorts of incursions. So — and as well, Poland, I think, traditionally has suffered a lot from Russian overreach, so not surprising that they would be worried right now."

A map highlighting Kaliningrad in red. https://www.google.com/maps/place/Kaliningrad/@54.1808003,25.4507782,6z/data=!4m2!3m1!1s0x46e33d8d4b7c21a9:0x5050960016126ed3

Kaliningrad was originally a part of Germany until its annexation by the Soviet Union in 1945. During the Cold War, Kaliningrad was one of the most militarized and closed off sections of the USSR.

Today, Kaliningrad is of extreme strategic value as it hosts the Russian Baltic Fleet in the port of Baltiysk — Russia's sole ice-free European port. Kaliningrad is also home to the Chernyakhovsk and Donskoye air bases. It is uncertain how many soldiers Russia has in the region; however, short-range mobile ballistic missiles have also been deployed in Kaliningrad since at least 2012.

Poland has been on edge ever since Russian accusations that it had set-up and encouraged militant training camps for Euromaidan protestors. According to these allegations, snipers during the protests were acting under Polish and American orders.

A alleged Russian saboteur has already been detained in eastern Ukraine. He is being charged with planting explosives and preparing other acts of diversion in an attempt to justify Russian action in the region.

Last week, fearing a possible spread of the Ukrainian conflict, Poland invoked a NATO rule allowing for military consultations between allies if it felt under threat.

The United States has since increased military exercises with Poland. It has also supplied additional aircraft to the Baltic States to augment a NATO patrol force there.