Just 18 months ago, White House and Pentagon officials debated whether Russia’s forces in Ukraine might collapse and be pushed out of the country entirely.

Now, after months of slow Russian ground advances and technological leaps in countering American-provided arms, the Biden administration is increasingly concerned that President Vladimir V. Putin is gathering enough momentum to change the trajectory of the war, and perhaps reverse his once-bleak prospects.

In recent days, Moscow’s troops have opened a new push near the country’s second-biggest city, Kharkiv, forcing Ukraine to divert its already thinned-out troops to defend an area that it took back from Russian forces in a stunning victory in the fall of 2022.

Artillery and…