One of the State Department's top nuclear experts resigned this week after President Donald Trump made the controversial decision to withdraw the US government from the Iran nuclear deal.

Trump's tenure has fostered a fair amount of drama at the State Department thus far, most evident in the recent replacement of Rex Tillerson, with Mike Pompeo as secretary of state.

The president's decision on the Iran deal has placed the US government at odds with key European allies, including the UK, France and Germany, who described the move as regrettable.

One of the State Department's top nuclear experts resigned this week after President Donald Trump made the controversial decision to withdraw the US government from the Iran nuclear deal.

Richard Johnson, acting assistant coordinator in the State Department's Office of Iran Nuclear Implementation, announced his resignation in an email to colleagues, which was obtained and first reported by Foreign Policy.

In the email, Johnson did not give an explicit reason for his departure, though he did state the Iran nuclear deal had "clearly been successful," a perception clearly not shared by Trump, based on the president's decision.

"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.

One official reportedly told Foreign Policy that Johnson did not plan on leaving the agency until Trump announced his Iran deal decision and another official described his departure as a "big loss."

Johnson's resignation is part of a troubling trend as Trump's tenure has fostered a fair amount of drama at the State Department thus far, most evident in the recent replacement of Rex Tillerson with Mike Pompeo as secretary of state. Moreover, roughly 60 percent of the department's top-ranking career diplomats have departed under Trump.

Meanwhile, the president's decision on the Iran deal has placed the US government at odds with key European allies, including the UK, France and Germany, who described the move as regrettable. It's not yet clear how Iran will respond, though Iranian President Hassan Rouhani responded to Trump's decision with a warning his country could resume enriching uranium within weeks.

Through Trump routinely referred to the Iran nuclear deal as "terrible," there's no credible evidence Tehran has violated the terms of the agreement. The deal, orchestrated by the Obama administration and finalized in 2015, was designed to prevent Iran from developing nuclear weapons in exchange for the easing of harsh economic sanctions.