Trump's reckless 'leadership' is driving a mass exodus from the State Department and leaving America unprepared for the diplomatic challenges we face.

One of the State Department’s top nuclear experts abruptly resigned this week following Trump’s decision to pull out of the Iran nuclear deal and leave the U.S. in violation of the international agreement.

Richard Johnson, acting assistant coordinator in the agency’s Office of Iran Nuclear Implementation, announced his decision to leave in an email to his colleagues this week, according to Foreign Policy, which obtained his resignation email.

Johnson had been involved in talks with key U.S. allies working to save the Iran deal in recent weeks, but ultimately those efforts failed.

On Tuesday, Trump announced that he had decided to back out of the pact, a move that was described as “calamitous” and as a “major strategic mistake.”

Johnson stopped short of explicitly stating that he was resigning because of Trump, but his email made it clear that Trump’s decision to violate the terms of the international deal was weighing heavily on his mind.

“I am proud to have played a small part in this work, particularly the extraordinary achievement of implementing the [deal] with Iran, which has clearly been successful in preventing Iran from acquiring a nuclear weapon,” Johnson wrote.

State Department officials told Foreign Policy that Johnson’s departure was unexpected. One official said Johnson had not planned on leaving the agency until Trump announced the Iran deal decision, and another described his exit as a “big loss.”

Johnson’s resignation is just the latest sign of trouble in the State Department under the Trump administration, reflecting a “brain drain” in the agency that has resulted in a mass exodus of career diplomats and civil servants. About 60 percent of top-ranking career diplomats have fled the agency since Trump took office.

In late March, more than 200 retired career diplomats signed a letter sounding the alarm about the corrosion of the State Department, warning that the Trump administration’s neglect of the institution has “crippled” its capacity.

At the time, eight of the nine senior leadership positions in the State Department were unfilled, 50 ambassadorships were vacant, and 16 of the 22 assistant secretary positions were empty.

Among the vacant posts were senior positions involving nuclear nonproliferation and arms control, along with the lead coordinator overseeing implementation of the Iran nuclear deal — a position that Trump never even filled.

Meanwhile, Trump’s decision to exit the Iran deal has made the rest of the world lose trust in American leadership and placed us at odds with some of our closest allies, increasing the need for effective diplomacy just as he guts our capacity to engage in it.