Russian troops’ failure to quickly seize Ukraine, however, has shown Mr. Shoigu’s changes, while real, didn’t create the fearsome fighting force he touted. Poor logistics, flawed strategy and ill-prepared troops mean any victory will be immensely costly, and an occupation hard to sustain.

Experts on the Russian military place some of the blame on Mr. Shoigu’s willingness to back Mr. Putin’s plans, even if they are unrealistic. That has meant agreeing with assumptions the Ukrainian military would quickly fold in the face of a superior force and that Russian troops would be greeted as liberators.

“Beyond Putin, this is very damning for one person in particular, and that is Sergei Shoigu,” said Michael Kofman, director of Russia studies at CNA, a nonprofit research and analysis organization located in Arlington, Va. “By agreeing to these assumptions and this type of operation, he has essentially thrown the Russian military into a disaster.”