WASHINGTON — When President Trump’s national security adviser, Robert C. O’Brien, convenes meetings with top National Security Council officials at the White House, he sometimes opens by distributing printouts of Mr. Trump’s latest tweets on the subject at hand.

The gesture amounts to an implicit challenge for those present. Their job is to find ways of justifying, enacting or explaining Mr. Trump’s policy, not to advise the president on what it should be.

That is the reverse of what the National Security Council was created to do at the Cold War’s dawn — to inform and advise the president on national security decisions. But under Mr. O’Brien, the White House’s hostage negotiator when Mr. Trump chose him to succeed John R. Bolton in September, that dynamic has often been turned on its head.

Mr. O’Brien, a dapper Los Angeles lawyer, convenes more regular and inclusive council meetings than Mr. Bolton. But developing policy is not really Mr. O’Brien’s mission. In the fourth year of his presidency and in his fourth national security adviser, Mr. Trump has finally gotten what he wants — a loyalist who enables his ideas instead of challenging them.