In the last days of 2019, Gen. Mark A. Milley, the Joint Chiefs of Staff chairman, Defense Secretary Mark T. Esper and senior Pentagon officials gave President Trump a list of options for responding to Iranian-led violence in Iraq.

They included an extreme one — killing Maj. Gen. Qassim Suleimani, Iran’s most powerful commander — as almost an afterthought, convinced the president would not take it.

He did. On Jan. 3, 2020, the Iranian general was killed in a U.S. drone strike near Baghdad International Airport.

The fallout was immediate. Iranian groups put a price on General Milley’s head. He, along with Mr. Esper and the Central Command leader, Gen. Kenneth F. McKenzie Jr., moved to the top of Iran’s retaliatory kill list, U.S. officials have said.

Now, a decision by Defense Secretary Pete Hegseth to remove General Milley’s security detail has raised alarm as President Trump seeks retribution against his perceived enemies at home.

Even Mr. Trump’s allies are concerned.

“I would encourage the president to revisit this,” Senator Tom Cotton, Republican of Arkansas and the chairman of the Senate Intelligence Committee, said on Wednesday.