DNIPROPETROVSK REGION, Ukraine — The quality of Ukraine’s military force, once considered a substantial advantage over Russia, has been degraded by a year of casualties that have taken many of the most experienced fighters off the battlefield, leading some Ukrainian officials to question Kyiv’s readiness to mount a much-anticipated spring offensive. U.S. and European officials have estimated that as many as 120,000 Ukrainian soldiers have been killed or wounded since the start of Russia’s invasion early last year, compared with about 200,000 on the Russian side, which has a much larger military and roughly triple the population from which to...