The question of whether President Trump leveraged military assistance to Ukraine for personal gain is at the heart of the impeachment inquiry. Today, we speak with our Ukraine correspondent on why that assistance was so important to Ukraine — and the United States — in the first place.Guest: Andrew E. Kramer, who covers Ukraine for The New York Times and is based in Moscow. For more information on today’s episode, visit nytimes.com/thedaily. Background reading: Petro O. Poroshenko, who was Ukraine’s president until May, knew his country’s independence hinged on American support. So he waged a campaign to win over President Trump.As vice president, Joe Biden tried to press Ukraine’s leaders to clean up corruption and reform the energy industry. The story of that effort has been overtaken by his son’s work for a Ukrainian gas company.