PRESIDENT TRUMP’S disclosure of highly classified information to senior Russian officials was the most disturbing demonstration yet that he is dangerously unprepared to handle sensitive national security matters. On Tuesday, Mr. Trump essentially confirmed a Post report that he provided details of the Islamic State’s plotting of airline attacks to Russian Foreign Minister Sergei Lavrov and Ambassador Sergey Kislyak in an Oval Office meeting last week. The Post reported that the information could allow Moscow to identify the source of the intelligence, which came through a foreign government with which U.S. spy agencies have a special relationship.

The consequences of the president’s lapse could be far-reaching. In addition to disrupting a vital flow of intelligence and possibly endangering agents on the ground, Mr. Trump has let the world know that he and his administration cannot be trusted with sensitive information. Governments that share their secrets with the CIA, from Britain to Israel — which was reported to be the source of the information — may feel compelled to recalibrate their cooperation. Those that don’t have a cooperative relationship, such as Russia and China, will try to use their access to Mr. Trump to extract more indiscretions.

The administration’s attempts to defend the leak only underlined the continuing chaos in the White House. When the Post article first appeared Monday, senior administration officials issued denials: National security adviser H.R. McMaster and deputy adviser Dina Powell both called it “false.” Mr. Trump then undercut them by confirming on Twitter that he provided the Russians with “facts pertaining . . . to terrorism and airline flight safety,” which, he said, “I have the absolute right to do.” By midday Tuesday, Mr. McMaster found himself simultaneously arguing that he was right to call the article false and spinning the president’s leak as “appropriate.”

In fact, everything about Mr. Trump’s engagement with the Russian officials reflected the gross inadequacy of his knowledge of foreign affairs as well as the weakness of the staff and processes he has put in place to aid him. His decision to meet with the chronically dishonest Mr. Lavrov and with Mr. Kislyak, who already had several questionable contacts with senior administration officials, itself reflected poor judgement; the Obama administration had refused to give Mr. Lavrov an Oval Office meeting since 2013. As the meeting began, U.S. journalists were banned from the room, while a Russian news-agency photographer was invited in, producing embarrassing photos and raising the possibility of a security breach.

Mr. Trump’s subsequent disclosures appeared to flow from two of his deepest flaws, vanity and an obtuseness about the regime of Vladi­mir Putin. As The Post reported it, he appeared childishly boastful about his “great intel.” And as the president subsequently described it, he was hoping the Russians would respond to the leak with greater cooperation with U.S. operations against the Islamic State, as opposed to using it to undermine them.

(Bastien Inzaurralde,Monica Akhtar/The Washington Post)

That was a naive and dangerous conclusion, as any CIA briefer would have told the president. Unfortunately, Mr. Trump doesn’t pay much heed to intelligence professionals, even as he misuses their materials, endangers their operations and impugns their professionalism. The processes in place to ready the president for interactions with foreign leaders are shockingly attenuated: A lot of the spade work is done by his inexperienced son-in-law, Jared Kushner, while key positions at the National Security Council and State Department remain unfilled. As the president prepares for his first trip abroad later this week, including meetings with key Middle Eastern and European allies, the potential for further gaffes — and damage to key U.S. alliances — is alarmingly high.

Sen. Bob Corker (R-Tenn.) aptly described Mr. Trump’s presidency on Monday as “in a downward spiral.” Arresting the fall would require a thorough revamping of White House staffing and function, one that replaces disorder and ignorance with discipline and competence. That, in turn, would require corrective action by Mr. Trump — for which the nation can only hope.