During more than three years of Russia’s full-scale war on Ukraine, the front line has also served as a kind of vicious, real-time classroom.

Both Ukraine and Russia have made, and learned from, mistakes. So, too, has North Korea — which last fall sent 11,000 elite soldiers to support Russia’s military in the Russian region of Kursk, where Ukrainians had made a surprise incursion last summer.

North Korean troops’ progress — especially in drone warfare — has potential implications not only for Russia’s war on Ukraine but also peace on the Korean Peninsula.

Across the border from Kursk, in Ukraine’s northeastern Sumy region, Ukrainian soldiers who battled those soldiers described how the North Koreans went from suffering massive losses to learning electronic warfare.

“They went from using World War II tactics to managing on the battlefield with drones,” Capt. Oleh Shyriaiev, commander of the 225th Separate Assault Brigade, told NPR. “And they learned very quickly.”

Russia has now regained control of nearly all of Kursk. As ceasefire efforts stall, Ukraine’s defense intelligence has already warned that Russia could deploy North Korean soldiers in a new ground offensive this summer. Ukrainian President Volodymyr Zelenskyy told reporters that Russia is amassing 50,000 troops along the border with Sumy. The region’s governor Oleh Hryhorov said Russia has already captured several Ukrainian villages along the border.