For nearly five hours last Monday, Elon Musk’s government cost-cutting team joined with private security and with law enforcement authorities for an extraordinary showdown at an iconic building alongside the National Mall.

They were demanding access to the U.S. Institute of Peace. The group, a nonprofit created by Congress 41 years ago, supports diplomatic solutions to global conflicts, but on this day it became the hub of a bitter dispute with implications for U.S. constitutional law.

The confrontation stemmed from the White House’s decision this month to orchestrate the ouster of the institute’s president and top staff. When they refused to leave, a State Department official, alongside Mr. Musk’s team, the Department of Government Efficiency, moved to take control. By the end of the night, the office was occupied by new staff and a president backed by the Trump administration.

A New York Times reconstruction of the takeover — based on reporting from the scene as well as photographic evidence, interviews and court records — underscores the aggressive moves that the White House and Mr. Musk’s team have employed to assert power over Washington institutions. In this case, DOGE staff allegedly threatened to cancel all federal contracts held by the organization’s private security firm if it did not help them gain entry to the building and used the local police to assist them.

Now the matter is in court. At issue is whether President Trump had the power to take over an independent organization, albeit one whose board includes government officials and others confirmed by the Senate. The fired staff members said in the lawsuit that it was a “lawless assault,” but the administration argues that the institute is part of the executive branch and under the president’s authority.

The institute’s independence was clearly delineated in the law that established it, signed by President Ronald Reagan at the height of the Cold War. It mandated that the organization “promote international peace and the resolution of conflicts among the nations and peoples of the world without recourse to violence.” It trains peace negotiators and diplomats and briefs Congress.