The Obama administration, American allies, the International Monetary Fund and Ukrainian anti-corruption activists, among others, had all made clear that they were displeased with the performance of Viktor Shokin, who became prosecutor general in 2015. There is no evidence of wrongdoing by either Joe or Hunter Biden.
Trump faces an impeachment probe in the Democratic-led House over his urging the Ukrainian President in a July phone call to investigate the Bidens. That phone call led to a whistleblower complaint that in part alleges Trump withheld US aid to Ukraine over the matter. The President has denied doing anything improper.