President Donald Trump suffered a big political blow on Monday, barely a month into office, when his National Security Adviser, Michael Flynn, resigned over his Russia contacts.

Mr. Flynn, a close aide of Mr. Trump, admitted that he had “inadvertently” briefed Vice-President Mike Pence with “incomplete information” about his phone conversation with the Russian ambassador in Washington, Sergey Kislyak.

The allegation is that Mr. Flynn discussed American sanctions on Russia with Mr. Kislyak in the waning days of the Obama presidency and told him that Russia should wait till Mr. Trump’s inauguration.

He later denied speaking of the sanctions, and based on his brief, Mr. Pence publicly defended him. But after the media reported that they had sources vouching that Mr. Flynn had discussed the sanctions with the envoy, it became impossible for the White House to defend him.

Technically, Mr. Flynn’s calls with the Russian ambassador before he became part of the government are a breach of an 18th century law, the Logan Act, that makes it illegal for private individuals to conduct foreign policy. The context is grave for the Trump administration.

There are already allegations that Moscow interfered in the presidential elections in favor of Mr. Trump and that the Russians have some compromising personal information about Mr. Trump.

The resignation, however, is unlikely to contain the scandal. It raises even more questions about administration officials’ dealings with Russia and the way the government functions. Mr. Flynn, for example, already faces allegations that he acted with the knowledge of others in Mr. Trump’s transition team, and his past Russian links are being probed. If the scandal widens, it could derail Mr. Trump’s Russia reset plans. He could have avoided this early embarrassment had he paid more heed to those who questioned his picks for top jobs in the administration. Mr. Flynn, who was fired by President Barack Obama in 2014 as head of the Defense Intelligence Agency, was particularly unpopular in Washington. Mr. Trump’s other picks, be it Attorney General Jeff Sessions who faces allegations of racism, or Education Secretary Betsy DeVos who needed the Vice-President to cast a tie-breaking vote in the Senate for confirmation, are other cases in point. Such decisions cannot be unmade now. But Mr. Trump could learn some lessons from the Flynn episode. He could use better judgment when he chooses his next NSA. He should set his house in order and formulate a cohesive approach towards domestic and foreign policy issues, including stating clearly what his Russia policy is. If not, his administration could well be trapped in crisis mode.