Along with documents and interviews with a wide variety of people in Ukraine and the United States, the latest revelations show that Mr. Trump and Mr. Giuliani ran what amounted to a shadow foreign policy in Ukraine that unfolded against the backdrop of three elections — this year’s vote in Ukraine and the 2016 and 2020 presidential races in the United States…

Backed by Mr. Trump, Mr. Giuliani, who once aspired to be secretary of state, sought to tar Mr. Biden with unsubstantiated accusations of impropriety, while he and associates working with him in Ukraine on the president’s agenda pursued their own personal business interests.

With the political landscape scrambled by Mr. Poroshenko’s defeat in April and the arrival of a new cast of Ukrainian officials, the approach pursued by Mr. Giuliani and Mr. Trump undercut official United States diplomacy.

And the signals sent by Mr. Trump — long skeptical of the strategic value of backing Ukraine against Russia, its menacing neighbor to the east — complicated efforts by the new Ukrainian government to fortify itself against Moscow.