KYIV, Ukraine—Winter is approaching on the Ukrainian steppe. As temperatures fall, waves of chilling rain follow, dissolving roads and fields, turning them to mud that mires men and equipment. Then comes the deep freeze and snow, hardening the ground but making it tougher to fight. Armies on both sides of the war in Ukraine are girding for the unforgiving weather and shorter days of winter, which will affect the health and morale of troops, diminish the effectiveness of weapons and intelligence-gathering sensors and multiply the logistical difficulties of keeping soldiers in the field. “Land warfare is generally very difficult. Winter...