PRESIDENT TRUMP and his defenders have been arguing, weakly, that his actions toward Ukraine, including demands for the investigation of his political opponents, were somehow consistent with U.S. national interests. There is no way to make that case about his treatment of Marie Yovanovitch, the former U.S. ambassador to Ukraine. In compelling testimony during the House’s impeachment inquiry on Friday, she described how the president’s firing of her was orchestrated by corrupt Ukrainian actors whom the United States had been trying to neutralize — and how that reversal damaged U.S. diplomacy around the world.

No wonder, then, that Mr. Trump, who has never offered a reason for yanking Ms. Yovanovitch, took to Twitter to abuse her, claiming that “everywhere Marie Yovanovitch went turned bad.” Beyond the absurdity of suggesting that Ms. Yovanovitch somehow brought about the troubles of Somalia or Uzbekistan, Mr. Trump was attacking a still-serving federal government employee even as she testified to his wrongdoing. Democrats were right to suggest this amounted to witness intimidation.

The truth is that Ms. Yovanovitch was having an impact in Kyiv. As other witnesses have testified, she was aggressive in pushing the Ukrainian government to fulfill its promises to tackle corruption, something it did not do. In particular, she tangled with Yuriy Lutsenko, the general prosecutor. Mr. Lutsenko responded by launching a smear campaign against her, in conjunction with two shady U.S. businessmen and Mr. Trump’s private lawyer, Rudolph W. Giuliani. Mr. Lutsenko won Mr. Giuliani over with false claims about Joe Biden. The businessmen, who had been seeking Ms. Yovanovitch’s ouster since 2018, got Mr. Trump’s ear by making a large contribution to a PAC supporting him.

As Ms. Yovanovitch put it, “individuals, who apparently felt stymied by our efforts to promote stated U.S. policy against corruption . . . were able to successfully conduct a campaign of disinformation against a sitting ambassador, using unofficial back channels. . . . They shared baseless allegations with the president and convinced him to remove his ambassador, despite the fact that the State Department fully understood that the allegations were false and the sources highly suspect.”

The consequences of this extend far beyond the humiliation of a distinguished diplomat who had served for 33 years. “Such conduct undermines the U.S., exposes our friends and widens the playing field for autocrats like President Putin,” Ms. Yovanovitch said. “Our Ukraine policy has been thrown into disarray, and shady interests the world over have learned how little it takes to remove an American ambassador who does not give them what they want.” Moreover, she said, the State Department has been degraded by the failure of its leadership — that would be Secretary of State Mike Pompeo — “to push back as foreign and corrupt interests apparently hijacked our Ukraine policy.”

Republicans tried to argue that Ms. Yovanovitch’s story had nothing to do with Mr. Trump’s campaign to have Ukraine launch political investigations. But Mr. Giuliani clearly saw Ms. Yovanovitch as an obstacle to his effort to orchestrate a probe of Mr. Biden. They suggested that, because Ms. Yovanovitch now has an academic fellowship, she had suffered no great harm. In reality, as she described it, attacks on her and other dedicated public servants are “leading to a crisis in the State Department as the policy process is visibly unraveling, leadership vacancies go unfilled, and senior and mid-level officers ponder an uncertain future and head for the doors.”

“How could our system fail like this?” the ambassador asked. “How is it that foreign corrupt interests could manipulate our government?” The answer lies with a president who put his personal interests — and those of corrupt Ukrainians — above those of the United States.

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