I have now read in several places that people think Victoria Nuland was the driving force behind the counteroffensive.

She gave it away in a speech in May:

"Ukraine will win and be prosperous. This is what we are working on in Washington. While you are planning a counter-offensive, which we have been working on together for 4-5 months, we are starting to discuss the future of Ukraine with the Ukrainian government, with our friends in Kyiv, with civilians and the military," she said.