“We must not allow the United States to become a country where standing up to our government is a dangerous act,” Yovanovitch wrote. “It has been shocking to experience the storm of criticism, lies and malicious conspiracies that have preceded and followed my public testimony, but I have no regrets. I did — we did — what our conscience called us to do. We did what the gift of U.S. citizenship requires us to do.”

In their congressional testimonies, impeachment witnesses described a smear campaign against Yovanovitch orchestrated by Rudy Giuliani, Trump’s personal attorney, who sought the envoy’s removal from her post in Kiev to ease the pursuit of foreign probes into the president’s domestic political rivals.

The House voted in December to impeach Trump on two articles, charging him with abuse of power and obstruction of Congress related to the Ukraine scandal. He was acquitted by the Senate on Wednesday.

Yovanovitch wrote Thursday that the events of the past year have “shown that we need to fight for our democracy” and emphasized that America’s public servants “need responsible and ethical political leadership.”

“This administration, through acts of omission and commission, has undermined our democratic institutions,” she wrote, “making the public question the truth and leaving public servants without the support and example of ethical behavior that they need to do their jobs and advance U.S. interests.”

Yovanovitch’s op-ed confirmed reports in recent days that she had retired from the State Department, concluding a three-decade career as a foreign service officer. She most recently served as a fellow at Georgetown University after being recalled from Ukraine.

“These are turbulent times, perhaps the most challenging that I have witnessed,” wrote Yovanovitch, who testified in November that her mother and father fled Soviet and Nazi regimes to immigrate to the U.S. “But I still intend to find ways to engage on foreign policy issues and to encourage those who want to take part in the important work of the Foreign Service. Like my parents before me, I remain optimistic about our future.”