For more than a decade, the United States has nurtured a secret intelligence partnership with Ukraine that is now critical for both countries in countering Russia.

Nestled in a dense forest, the Ukrainian military base appears abandoned and destroyed, its command center a burned-out husk, a casualty of a Russian missile barrage early in the war.

But that is above ground.

Not far away, a discreet passageway descends to a subterranean bunker where teams of Ukrainian soldiers track Russian spy satellites and eavesdrop on conversations between Russian commanders. On one screen, a red line followed the route of an explosive drone threading through Russian air defenses from a point in central Ukraine to a target in the Russian city of Rostov.

The underground bunker, built to replace the destroyed command center in the months after Russia’s invasion, is a secret nerve center of Ukraine’s military.

There is also one more secret: The base is almost fully financed, and partly equipped, by the C.I.A.

“One hundred and ten percent,” Gen. Serhii Dvoretskiy, a top intelligence commander, said in an interview at the base.

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