Toward the end of 2021, according to a senior European official, Mr. Putin was weighing whether to launch his full-scale invasion when he met with the head of one of Russia’s main spy services, who told him that the C.I.A., together with Britain’s MI6, were controlling Ukraine and turning it into a beachhead for operations against Moscow.

In Kyiv, Mr. Nalyvaichenko picked a longtime aide, General Kondratiuk, to serve as head of counterintelligence, and they created a new paramilitary unit that was deployed behind enemy lines to conduct operations and gather intelligence that the C.I.A.

The new station chief began regularly visiting General Kondratiuk, whose office was decorated with an aquarium where yellow and blue fish — the national colors of Ukraine — swam circles around a model of a sunken Russian submarine.

also oversaw a training program, carried out in two European cities, to teach Ukrainian intelligence officers how to convincingly assume fake personas and steal secrets in Russia and other countries that are adept at rooting out spies.

One day after General Kondratiuk was removed, a mysterious explosion in the Russian-occupied city of Donetsk, in eastern Ukraine, ripped through an elevator carrying a senior Russian separatist commander named Arsen Pavlov, known by his nom de guerre, Motorola.

In the southern Kherson region, which was occupied by Russia in the first weeks of the war, those partisan networks sprang into action, according to General Kondratiuk, assassinating local collaborators and helping Ukrainian forces target Russian positions.

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