The timing of the switch – coming days after Mr Trump confirmed he had accepted an invitation to meet Kim Jong-il for talks despite months of back-and-forth insult – led some to conclude Mr Tillerson was less keen on the meeting.
He was actually on a visit to Africa when Mr Trump made his announcement, effectively meaning he was out of the loop.
As CIA chief Mr Pompeo has steered clear of calls to scrap the landmark 2015 deal with Iran, which puts restrictions on its nuclear programme in exchange for the lifting of some sanctions.
But he has previously made it clear he is no fan of the controversial agreement.
He told the Foundation for Defense of Democracies last year “We need even more intrusive inspection.
“The deal put us in a marginally better place with respect to inspection, but the Iranians have on multiple occasions been capable of presenting a continued threat through covert efforts to develop their nuclear program along multiple dimensions, right?
“The missile dimension, the weaponisation effort, the nuclear component itself.”
Daryl Kimball, executive director at the Arms Control Association and a regular contributor to the 38 North website, said: “Tillerson was a moderating influence on the administration’s foreign policy and his departure may have significant implications for the administration’s approach on key issues, including the Trump-Kim summit and the future of the Iran deal.
“The reshuffling at State will likely increase the challenges of preparing for Trump’s high-stakes summit with Kim Jong Un.
“At best this meeting might be the launching point for a sustained negotiation on steps toward denuclearisation of the Korean peninsula and establishing a peace regime on the Korean peninsula.
“While Pompeo is probably more trusted by Trump than Tillerson was, he has been far more hawkish about blowing up the Iran nuclear deal even though it is working and on the prospect of “regime change” in North Korea.”
Mr Kimball suggested it would be foolhardy to pull the plug on the Iran deal in the same month that he was likely to ask the North Koreans to sign up to a similar arrangement.
He explained: “If Trump really wants to secure a deal with Kim Jong-un or use his May summit meeting to launch negotiations on the terms and timelines for North Korean denuclearisation, it would be foolhardy to decide that same month unilaterally discard the Iran nuclear deal, which Iran is complying with and which is effectively blocking Iran’s pathways to the bomb.
“The reality is that the US cannot unilaterally re-impose sanctions on Iran in an attempt to change the original terms of the agreement without violating the deal and opening the door for Iran to walk away what is an effective and vital non-proliferation agreement — and create a major breach without European allies who strongly support the Iran Deal.”