Forget about the 810-mile border Finland shares with Russia. NATO needs to be wary of Russia’s Kaliningrad Oblast. It will soon become more problematic for NATO.

Finland and Sweden applied to join the North Atlantic Treaty Organization on Wednesday. After receiving their application letters, NATO Secretary-General Jens Stoltenberg told reporters, “I warmly welcome the requests by Finland and Sweden to join NATO. You are our closest partners." This will, without doubt, further antagonize Russian President Vladimir Putin and result in more nuclear saber-rattling, but it also presents additional challenges.

The NATO expansion will encircle the Baltic Sea, leaving Russia’s Kaliningrad Oblast, an exclave nested between Lithuania and Poland, virtually isolated with only a railway connecting it to Belarus. While relatively unknown to many Americans, the oblast’s strategic importance has steadily increased with the successive enlargements of the European Union and NATO.

The oblast was formerly the northern part of the Prussian province of East Prussia. With the defeat of Nazi Germany in World War II, the territory was annexed as part of the Russian Soviet Federative Socialist Republic by the Soviet Union via the Potsdam Agreement of 1945. Today, the Kaliningrad Oblast is possibly the most militarized region in the world. According to a 2014 Russian Ministry of Defense report, there were over 225,000 military personnel stationed in an oblast approximately the size of Connecticut — that’s 35,000 more than the force used to invade Ukraine.

From a Russian perspective, the armed forces stationed there are a counterbalance to the expanding NATO alliance. They serve to protect national interests and secure the interests of the Russian and Russian-speaking diasporas in the immediate vicinity — and not limited to just the Kaliningrad Oblast, Russian "hybrid warfare" tactics have targeted Russian-speaking populations throughout the Baltic States and Poland.

From NATO's perspective, Russian military forces in the Kaliningrad Oblast pose a serious threat. Many consider the oblast an important bridgehead for the Russian military and navy to gain control over the Baltic States and the Baltic Sea. This piece of key terrain presents a threefold threat.

First, the Baltiysk Naval Base in Kaliningrad is the home of the Russian Baltic Fleet and the 336th Naval Infantry Brigade. Their ability to interdict shipping lanes and shut down the Baltic Sea would significantly affect the region economically and prevent NATO from reinforcing the Baltic member states in the event of a conflict.

Second, there's the Suwalki Gap, a 40-mile-wide stretch of border between Poland and Lithuania. Its two highways are the only land corridor connecting the NATO countries. Russian forces from the west and Belarusian forces from the east could possibly seize the Polish Suwalki region to cut off the Baltic States from receiving military land support. Russia’s 1st Army Corps, 18th Guards Motor Rifle Division, 79th Motorized Guard Brigade, 11th Tank Regiment, and 7th Motorized Guard Infantry Regiment make up the bulk of those ground forces. These are armor heavy formations, composed of T-72 tanks, armored personnel carriers (BMP-2), and an abundance of artillery.

Third, Kaliningrad hosts the 152nd Guard Missile Brigade, which has a dozen two-shot Iskander-M ballistic missile systems. Counting reloads, the brigade can fire two volleys of 24 hypersonic ballistic missiles, with a range of nearly 310 miles, capable of striking northern Berlin. Russian tactical nuclear weapons stored in Kaliningrad are likely meant for use with this weapon system, which can carry up to a 50-kiloton nuclear warhead as well as a conventional warhead. This capability is exercised often; there have been regular military exercises involving Russia’s Baltic fleet for years, most recently Zapad-21 in September 2021.

To protect this arsenal from NATO, the 44th Air Defense Division blankets the oblast with an integrated air defense network consisting of eight battalions of long-range S-300 and S-400 surface-to-air missile batteries, as well as shorter-range Pantsir systems. The S-400 can launch 40N6E missiles with a maximum range of 250 miles; Russian SAMs could theoretically threaten aircraft over two-thirds of Polish airspace.

From its position in the Kaliningrad Oblast, Russia has the ability to influence the Baltic Sea region negatively — commercially and militarily. While the nuclear rhetoric continues to be worrisome, Russia’s conventional capability should not be overlooked in regard to its failures in Ukraine. How Russia reacts to NATO’s expansion will likely be seen in the Kaliningrad Oblast first.

Retired Army Col. Jon Sweet (@JESweet2022) served 30 years as a military intelligence officer. His background includes tours of duty with the 101st Airborne Division (Air Assault), the Defense Intelligence Agency, the National Security Agency, and the National Geospatial-Intelligence Agency.