Former National Security Adviser John Bolton told "Special Report" in an exclusive interview Tuesday that his resignation from the Trump administration was spurred by the president's "erratic behavior" which he said made it "harder to protect America and make America secure."

"I think for many others who have also resigned or left the White House or the administration, you work as hard as you can to provide the advice you think is best," Bolton told host Bret Baier.

BOLTON CALLS TRUMP CORONAVIRUS RESPONSE 'INCOHERENT' 

"There just comes a point where your political capital has been expended, where it’s clear the president is not inclined to listen."

Bolton disclosed that while he "had a letter of resignation written for quite some time in advance," the "final straw was ... the president’s idea of inviting the Taliban to Camp David to sign the so-called Afghan Peace Agreement [in September 2019], which I thought was a bad agreement. I thought the president was pursuing the wrong policy, and I thought that the decision to bring them to Camp David was just – I just couldn’t respond to it."

Bolton also told Baier that President Trump's policy on Iran "wasn't that different" from Barack Obama's in the first 14 months of the administration.

"When I joined in April of 2018, we were still in the Iran Nuclear Deal," said Bolton, who added that the U.S. withdrawal from the deal in May of that year "was my happiest moment, probably of the whole time" in the administration.

"I probably should have left the administration then," he said.

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Bolton went on to claim that Trump undercut his own hard-line Iran policy, telling Baier, "we did put enormous pressure on Iran but when it came down to it by near the end of my tenure, Trump’s still looking for that deal, wanted to meet with the Ayatollahs, wanted to meet with [Iran Supreme Leader] Khamenei, wanted to meet with the Iranian Foreign Minister [Javad Zarif]."

Bolton said Trump's "mixed signals" made it "very, very difficult if not impossible to really affect the kind of change that was necessary in Tehran."