WASHINGTON -- For the last 18 months, Defense Secretary Jim Mattis and other top national-security officials have mostly kept their heads down in public as they tried to quietly counsel President Trump. But this low-key consultation process seems to be weakening, as a headstrong president becomes increasingly insistent about his judgment.

The Helsinki summit showed that Trump thinks he's his own best foreign-policy adviser. The formal interagency process that traditionally surrounds such big events all but disappeared for the U.S.-Russia encounter, with no full National Security Council meetings to prepare for Helsinki and none last week to discuss its results.

Trump chose to go it alone with Russian President Vladimir Putin, literally and figuratively. Officials who wanted to know what happened had to read the president's tweets or White House press briefings.

"I don't think there is an interagency process now," cautioned one prominent Republican foreign-policy expert. "Trump glories in not listening to advisers. He trusts his instincts, as uninformed as they sometimes are."

Newt Gingrich, the former House speaker and one of Trump's closest informal advisers, outlined the challenge in an interview Tuesday: "A major problem for Trump is that he's a golfer; he doesn't play a team sport. The rest of the team has to know ... what play you're calling. In golf, it's just between you and the ball. I think that's a major weakness. ... The world is far too complicated for one person to control everything."

"It worries me," added Gingrich, that Trump doesn't consult more closely about Russia-related issues with officials such as Gen. Joseph Dunford, chairman of the joint chiefs of staff. As of Monday, Dunford still hadn't been briefed on Helsinki, even though it directly affects the more than 1 million troops Dunford oversees.

Gingrich is unusual among Trump insiders in his willingness to discuss the president so openly. Most top officials are gun-shy. They fear that Trump will see criticism as disloyalty and purge dissenters, as he did former Secretary of State Rex Tillerson. "Trump has tremendous confidence in you when you agree with him, and he doesn't think about you when you don't," noted Gingrich. "This is a president who gets bored," he said. The trick is "being near him, but not too near."

How should Trump's senior advisers deal with a president who operated in Helsinki as a one-man show? That varies with the different players.

Mattis, a retired four-star general who commands nearly universal respect in Congress, has an especially delicate challenge. During Trump's first year, Mattis was an effective inside player. He convinced Trump that advocating torture was a mistake, and that even a limited military strike on North Korea could have dangerous, unpredictable outcomes. According to Gingrich, Mattis's discussions with Trump about the catastrophe of nuclear war helped convince the president to embrace an arms-control initiative with Putin.

Trump is fickle, and he may now be chafing at his defense secretary's careful counsel. A Republican source tells me Trump recently complained: "He's not 'Mad Dog Mattis,' he's 'Moderate Mattis.'" Another Republican says that at the G7 summit in Canada in June, Trump asked European leaders, "What do you think of Mattis?" Europeans feared it was a trick question, because too much enthusiasm might undermine Mattis.

Trump has given Mattis the freedom to run the Pentagon. Mattis, in return, has been almost agonizingly discreet, saving his counsel for the president's ears. Some Mattis supporters wish he would lean forward a bit more -- but not if that exposes him to the president's ire and undercuts his essential role.

Secretary of State Mike Pompeo is the "Trump whisperer," the member of Trump's inner circle who seems most similar to him in outlook and temperament. He's the chief action officer on North Korea and Iran, the administration's two most ambitious, dangerous challenges. Will Trump give Pompeo the latitude to communicate strategy on these big issues to the country and the world? If he feuds with Pompeo, Trump is sunk.

The final piece of the post-Helsinki puzzle is National Security Adviser John Bolton. He is deliberately tightening the NSC circle, reducing the frequency of formal principals' meetings, to remedy what he thinks was interagency overkill during the Obama years. Bolton prefers small group meetings with Mattis, Pompeo and Trump. He mistrusts the bureaucracy, but will need to use it to cope with so many hotspots at once. Like most national security advisers, he wants to emulate Brent Scowcroft -- but he should recognize that Scowcroft was a master bureaucratic player.

Gingrich is right: Trump, the solo golfer, won't succeed unless he plays with the other members of his foursome. On his own, he won't make the cut.

(c) 2018, Washington Post Writers Group