On a tactical level, the partnership yielded triumph upon triumph. Yet at arguably the pivotal moment of the war — in mid-2023, as the Ukrainians mounted a counteroffensive to build victorious momentum after the first year’s successes — the strategy devised in Wiesbaden fell victim to the fractious internal politics of Ukraine: The president, Volodymyr Zelensky, versus his military chief (and potential electoral rival), and the military chief versus his headstrong subordinate commander. When Mr. Zelensky sided with the subordinate, the Ukrainians poured vast complements of men and resources into a finally futile campaign to recapture the devastated city of Bakhmut. Within months, the entire counteroffensive ended in stillborn failure.
The partnership operated in the shadow of deepest geopolitical fear — that Mr. Putin might see it as breaching a red line of military engagement and make good on his often-brandished nuclear threats. The story of the partnership shows how close the Americans and their allies sometimes came to that red line, how increasingly dire events forced them — some said too slowly — to advance it to more perilous ground and how they carefully devised protocols to remain on the safe side of it.
Time and again, the Biden administration authorized clandestine operations it had previously prohibited. American military advisers were dispatched to Kyiv and later allowed to travel closer to the fighting. Military and C.I.A. officers in Wiesbaden helped plan and support a campaign of Ukrainian strikes in Russian-annexed Crimea. Finally, the military and then the C.I.A. received the green light to enable pinpoint strikes deep inside Russia itself.