As Russians approached Kyiv and Kharkiv, the Ukrainians were able to shift their forces to critical locations faster than the invading forces. Not only have the Ukrainians moved more nimbly, they also made good choices about where to concentrate firepower.

“The art of mechanized maneuver warfare is being able to concentrate overwhelming combat power at decisive sections of the front, places of your choosing,” said Frederick W. Kagan, a military strategist who has advised the U.S. command in both Iraq and Afghanistan. “The Russians, astonishingly, failed to do that. But the Ukrainians have taken advantage of their ability to move reinforcements rapidly and counterattack.”

Thomas Bullock, an open-source analyst from Janes, the defense intelligence firm, said Russian forces have made tactical errors that the Ukrainians have been able to capitalize on.

“It looks like the Ukrainians have been most successful when ambushing Russian troops,” Mr. Bullock said. The Russians “have stuck to main roads so that they can move quickly and not risk getting bogged down in mud. But they are advancing on winding roads and their flanks and supply routes are overly exposed to Ukrainian attacks. And that is where they have had their most success.”