Shortly after Russia invaded Ukraine in 2022, Putin’s forces began assembling a series of complex defensive fortifications known as the Surovikin Line—minefields, trenches, ditches, and concrete “dragon’s teeth” covering dozens of square miles of Ukrainian territory. For a time, the two countries were locked in a horrific war of attrition that seemed like it would inevitably end in victory for Moscow. Corey Wilkens, a sergeant major with the U.S. Army combat engineers, recalled the intense trepidation he felt studying the Surovikin Line from his post at Fort Bragg. “Not only would we have a really, really hard time being able to breach that in general, but with our current methodologies, we would have mass casualties of our engineer units if we had to go do that,” he told me.