Why Russia's defenses are so effective: Deep minefields, adaptable drones, and electronic warfare are proving formidable in Ukraine, experts say.

David Lewis, a senior associate fellow at the think-tank Royal United Services Institute (RUSI), told Insider that Russia's defensive lines are solid because of its calculated withdrawal from the city of Kherson in November last year. "That decision has proved tactically correct from Moscow's point of view because it allowed Russian forces to defend a relatively stable front line, using extensive fortifications and defensive lines — and with the protection of the Dnipro river in the west," he told Insider. "The natural advantages of a defensive force have worked in Russia's favor," he added.