The Russian military has faltered early in its invasion of Ukraine, as stiff resistance threatens to turn Moscow’s hopes for a swift victory into a protracted and costly war, U.S. officials and allied military experts say.

The Kremlin’s ongoing invasion represents the most formidable challenge for the Russian military since it was modernized under Gen. Valery Gerasimov, Russia’s top military officer who was appointed to his post in 2012. Yet no Ukrainian cities have been taken by the Russian military. Some of the Ukrainian Air Force and air defenses are still intact.

Western officials and analysts say that Russia’s strategy had been based on the premise that an initial barrage of missile strikes and a thrust toward Ukraine’s capital would bring about the quick collapse of President Volodymyr Zelensky’s government. Mr. Zelensky, however, remains defiant while facing long odds.