The most dangerous occupation in Washington these days is being an expert.

Across West Executive Avenue from the White House, the offices that once buzzed with specialists at the National Security Council are now half vacant. Their dismissal reflects an administration not especially interested in the policy options developed by the specialists — many drawn from the State Department or the C.I.A. — who stayed deep into the night pressure-testing alternatives to immediate and long-simmering crises.

As Karoline Leavitt, the White House press secretary, told reporters this week, the old structure did not fit the “top-down approach” of President Trump, who views the institution’s proper role as carrying out his decisions.

At the Pentagon, 14 advisory boards have been dismantled, with curt, thank-you-for-your-service notes sent to Democrats and Republicans alike. Some of the boards dealt with obscure matters. But others focused on vital issues, like rethinking the American nuclear arsenal as China’s nuclear buildup, Vladimir V. Putin’s episodic nuclear threats and Mr. Trump’s ambitious demand for a “Golden Dome” missile defense system have changed the nature of nuclear strategy.

Also gone: The board of experts who were trying to learn lessons from China’s astoundingly successful hack into the country’s telecommunications networks — where, by all accounts, they remain to this day. Then came historians at the State Department and the climate specialists at the National Oceanic and Atmospheric Administration, which employed experts in weather, oceans, climate and biodiversity.

The National Weather Service lost so many people that the agency had to hire them back. No such luck for researchers relying on the National Science Foundation, where projects are disappearing every month.