US officials told Ukraine's President Zelenskiy in writing that to get US civilian aid he would have to publicly call for an investigation of Hunter Biden. They even sent him words he was supposed to say to the public.

Ukrainian officials were uncomfortable with this corrupt deal. They did not like being a tool for dishonest US domestic politics. I suspect that the bully had a feeling Ukraine would not go through with it and that he canceled US military aid to increase the pressure. But this backfired.