In the first century AD, two Roman centurions searching for sources of the Nile were halted by enormous marshes where the water was difficult to traverse even by small boat. The Sudd of present day South Sudan is a maze of channels, floating vegetation, mud and seasonally changing water, creating a formidable barrier even to this day. The account captures a durable strategic truth: sometimes the most effective obstacle is not a wall, but the ground itself.

Nearly two millennia later, NATO members are revisiting the same principle. Finland, Lithuania, Latvia and Poland are exploring ways to use environments as part of national defense. Their terrain-shaping measures include restoring selected landscapes, strengthening natural obstacles, narrowing an attacker’s choices and making geography work are becoming part of a larger defense-in-depth system.