The former U.S. ambassador to Estonia on Wednesday wrote that he resigned from his position earlier this year because he "could no longer support President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s policies."

James Melville, a career diplomat who served in the U.S. Foreign Service for more than three decades, wrote in an op-ed published in The Washington Post on Wednesday that he left his position because he believed it was "honorable and right" to disassociate himself from an administration pursuing an agenda he said he could no longer stand behind.

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Melville cited the president's antagonism toward NATO, trade policies and apparently warm relationship with Russian President Vladimir Putin as factors, which ultimately motivated his decision.

"Arrogance does not suit us well," Melville wrote, adding that the president refuses to work "in concert with our allies and partners."

"'America First' is a sham," Melville wrote.

A top agency official told Foreign Policy magazine in June that Melville's resignation was a blow to the agency and a sign of resistance to Trump's policies from within the diplomatic community.

"I had no choice but to resign," Melville wrote in the Post op-ed. "I have no sympathy or understanding for those who remain in government service while claiming to be ignoring or otherwise violating their instructions."

Melville, at the time of his departure, wrote on Facebook that he was frustrated with Trump's treatment of NATO allies.

Trump, in July, publicly questioned why the U.S. must commit to defending all NATO allies. During a NATO summit this summer, he pushed U.S. allies to "substantially up" their defense spending commitments.

"The alliances we have built and the friends we have made all over the globe look to the United States for leadership and guidance as we, together, tackle the world’s challenges," Melville wrote. "There should be no question that we are more effective and powerful when we work in concert with our allies and partners."

He added that he plans to use his "voice" to speak up for maintaining close ties with U.S. allies and seeking "global good."