REX TILLERSON was poorly cast as secretary of state. The veteran oil executive never grasped some of the essentials of diplomacy, including the importance of public communication. He badly damaged the State Department by ignoring its professionals, scores of whom departed, while conducting a prolonged and ill-advised reorganization. He disregarded American principles by playing down human rights, and he proved ineffective as a negotiator — whether with U.S. allies such as the Persian Gulf states or adversaries such as Russia.

None of those weaknesses, however, appear to explain why President Trump abruptly dismissed, via tweet, Mr. Tillerson on Tuesday — the last of many humiliations the president inflicted on his top diplomat. Rather, Mr. Trump suggested that he and Mr. Tillerson were “not really thinking the same” on key foreign policy issues, which is true. Mr. Tillerson resisted the president’s steps toward voiding the nuclear deal with Iran and the Paris climate treaty, and opposed moving the U.S. Embassy in Israel to Jerusalem. When Mr. Tillerson spoke in favor of diplomacy with North Korea, Mr. Trump tweeted that he was wasting his time; then, the same day Mr. Tillerson said negotiations with the regime of Kim Jong Un might be a long way off, Mr. Trump agreed to a summit meeting.

Most disturbingly, Mr. Trump would not back the tough stances that Mr. Tillerson struck, with ample reason, against the misdeeds of Russian ruler Vladi­mir Putin. Most recently the secretary said he agreed with the British government that Russia should be held responsible for an attack on British soil with deadly nerve gas. But Mr. Trump on Tuesday was equivocal, suggesting the administration still needed to “ get the facts straight.”

President Trump spoke March 13, after it was announced he ousted Secretary of State Rex Tillerson and planned to tap CIA Director Mike Pompeo to replace him. (The Washington Post)

Mike Pompeo, the CIA director and former member of Congress whom Mr. Trump nominated as his next secretary of state, is more in tune with the White House. He shares Mr. Trump’s contempt for the Iran nuclear deal, and he has spoken of regime change in North Korea as an objective. He reportedly has forged a rapport with Mr. Trump and has been better regarded as a manager at the CIA than Mr. Tillerson has been at State. At best, Mr. Pompeo could restore morale and professionalism at Foggy Bottom; a good start would be filling the dozens of important positions that remain vacant.

Mr. Pompeo will nevertheless start with some steep challenges. Mr. Trump is two months away from the next deadline for renewing the suspension of sanctions on Iran, and the summit meeting he committed to with North Korea was said to be possible by the end of May. Britain is looking to the United States for support in responding to the nerve-gas attack. On all those issues, Mr. Trump is courting disaster — from a confrontation with Tehran for which the United States is ill-prepared, to a rift with its closest ally.

Meanwhile, Mr. Pompeo and Mr. Trump’s nominee for CIA director, Gina Haspel, must still win Senate confirmation. Senators should have hard questions for both: Mr. Pompeo should be asked to explain how the administration can void the nuclear deal without triggering a breach with allies, or war with Iran, and what it can hope to gain from a Korean summit meeting. Ms. Haspel should account for her role in overseeing a CIA “black site” where detainees were tortured during the George W. Bush administration. As Mr. Trump’s conduct of foreign policy grows increasingly chaotic, some checking by Congress is more needed than ever.

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