In the past, American diplomats worked closely with Kiev in any talks with Moscow. They presented a united front to the Kremlin, cajoled the European Union to maintain sanctions and tried to reassure a nervous Ukrainian public. Kurt D. Volker, the State Department special envoy for Ukraine, traveled to the country frequently, held talks with Russian officials and agitated on behalf of Kiev at the White House, on Capitol Hill and in Europe.

But with American policy and personnel scrambled by revelations about Mr. Trump’s push for Ukraine to investigate Democrats, the United States is now largely absent from the political and diplomatic process over resolving the war in the east, Ukrainian and Western officials in Kiev say.

Since this spring, as Mr. Trump’s pressure campaign built toward its highest pitch, at least nine officials who had a hand in Ukraine policy have either resigned or become distanced from the Trump administration after testifying in the impeachment inquiry: Mr. Volker; John Bolton, the former national security adviser; Fiona Hill, the former adviser on Europe and Russia at the White House; Marie Yovanovitch, the former ambassador to Ukraine; Michael McKinley, a senior adviser to Secretary of State Mike Pompeo; Lt. Col. Alexander S. Vindman, the top Ukraine expert on the National Security Council; and the diplomats Gordon D. Sondland, William B. Taylor Jr. and George P. Kent.

Only Mr. Taylor is still active in Ukraine, serving as the acting United States ambassador. On Wednesday, Mr. Taylor testified to the House Intelligence Committee, which is leading the impeachment inquiry, that his aide was told in July that Mr. Trump cared more about “investigations of Biden” than he did about Ukraine.