As Ukrainian troops probe Russian defenses, Kyiv’s forces face an enemy that has made mistakes and suffered setbacks in the 15-month-old war. But Moscow also has learned from those blunders and improved its weapons and skills. Russia has built heavily fortified defenses along the more than 1,000-kilometer (600-mile) front line, honed its electronic weapons to reduce Ukraine’s edge in combat drones and used its massive Cold-War-era arsenal of heavy bombs to turn them into precision-guided gliding munitions capable of striking targets without putting its warplanes at risk. That could threaten to turn the fight into a long battle of attrition.