NICK SCHIFRIN:

Lieutenant Colonel Deric Holbrook leads the 2nd Cavalry's Field Artillery Squadron. Part of the Polish exercise is visiting small towns like Suwalki. Soldiers and fathers encouraged kids to become comfortable with guns.

Nearby, young baton twirlers entertained the crowd. It seemed like this entire town of 60,000 showed up in support. The next morning, Holbrook's men hit the road.

They were on a 500-mile drive, the longest military movement in Eastern Europe since World War II. They drove through this 60-mile-wide gap between NATO members Poland and Lithuania.

This is the Suwalki Gap, where NATO's notion of collective self-defense is perhaps most vulnerable. Thirty miles to my west is Kaliningrad, a Russian enclave. Thirty miles to my east, Belarus, a Russian ally. If Russia wanted to move into this gap, it could most likely cut off the Baltic states from the rest of Europe and the rest of NATO.

That geography is a product of NATO's expansion east. In 1949, NATO's eastern border was Italy. By 1999, it had added seven more countries, by 2009, nine more countries, including the Baltics. Suddenly, NATO had beachheads on Russia's borders.

DIMITRI SIMES, Center for the National Interest: If we would discover that there would be Russian troops in Mexico, I wonder how many American politicians and indeed American people would that think this is an acceptable situation.