The question, however, is why all this was done behind the backs of Kiev, the State Department and most everyone else. The plan seems to have originated with Andrii Artemenko, the Ukrainian legislator and a wheeler-dealer who sees himself as a Ukrainian Trump. He also claimed to have evidence of corruption that could bring down Ukraine’s president, Petro Poroshenko.

The Kremlin denied it had known anything about the plan. The Ukrainian government was livid, and prosecutors in Kiev are investigating whether Mr. Artemenko committed treason. Mr. Cohen dismissed reports about the plan as “fake news” and denied delivering anything to anyone, though he acknowledged meeting the other two men. Felix Sater, the third man — a New York real estate developer who once served prison time for a stock-fraud scheme and again for a bar fight and then cooperated with federal authorities investigating fraud cases — indignantly insisted he had had no contact with anyone in the Russian government on behalf of Mr. Trump or the plan.

President Vladimir Putin’s aggressive policies are among the major challenges facing the United States and Europe, and Ukraine is central to that challenge. Western steadfastness and unity are critical, yet the unanswered questions about Mr. Trump’s relations with Russia have caused considerable consternation among allies, which his lieutenants have been trying hard to assuage.

Mr. Cohen is Mr. Trump’s lawyer; he cannot pretend to be acting as a concerned citizen when he meets with opposition Ukrainian politicians bearing suspect proposals. This is no time for freelancing.