Durham, North Carolina, and Washington, DC (CNN) John Bolton issued a rebuke against White House "censorship" of his book manuscript in his first public remarks since the impeachment inquiry into President Donald Trump concluded, and expressed uncertainty about whether the public will ever have an opportunity to read what he described as his "effort to write history."

Bolton, who departed from his job in September following some policy disagreements with the President, reiterated some of those differences during the event at Duke University on Monday, but declined to corroborate any of the details offered by witnesses in the impeachment inquiry or divulge other details from his highly anticipated book, which is currently under review by the White House.

"I hope it's not suppressed," Bolton told the audience, referring to the White House's review of his book.

"I say things in the manuscript about what he (Trump) said to me," he added. "I hope they become public someday."

For weeks, Bolton and his lawyers have been embroiled in a battle with the White House over the contents of the book, which is due to be published in March -- the administration is raising concerns about the publication of classified information that it says is protected by executive privilege. The White House's records management office has been responsible for reviewing Bolton's book and providing feedback, although the office typically reviews manuscripts for information that is protected under executive privilege and often defers on classified matters to the National Security Council.

Though the book is being reviewed, the White House has not attempted to stop Bolton speaking publicly and has not asked him to avoid publicly discussing issues related to his time working for Trump.

The administration has argued much of the book can't be published due to the sensitivity of the content.

The former national security adviser's comments at Duke, the first of two public appearances this week, was billed as focusing on the national security challenges facing the country.

The former national security adviser wasn't expected to directly address the issue of impeachment or the ongoing conflict with the White House over the contents of his highly anticipated book. However, he frequently teased some of the information in his book and made several passing remarks about the pre-publication process delaying its publication.

Asked, at one point, whether he shares Trump's assessment that the July 2019 call with Ukraine's President Volodymyr Zelensky at the heart of his impeachment was "perfect," Bolton responded simply, "You'll love Chapter 14."

The New York Times reported last month that Bolton, in his unpublished manuscript, alleges that Trump directed him to help with his pressure campaign to get damaging information on Democrats from Ukraine. According to the Times, he also alleges that Trump didn't want to lift a hold on military aid to Ukraine until officials there complied with the request. Bolton's lawyers have not denied the substance of the New York Times reports but have condemned the leaks behind it.

"For all the focus on Ukraine and impeachment trial: to me there are portions of the manuscript that deal with Ukraine -- I view that as the sprinkles on an ice cream sundae, in terms of the book. This is an effort to write history. I did the best I can. ... We'll see what happens with the censorship," Bolton said.

A majority of Republican senators voted against calling witnesses -- including Bolton -- to testify during the impeachment trial, sparking criticism from Democrats who said there is more to be uncovered. House Democrats didn't subpoena Bolton during their inquiry that led to Trump's impeachment, claiming it would be challenged in the courts and slow the process.

Bolton is also due to appear alongside President Barack Obama's former national security adviser Susan Rice on Wednesday at Vanderbilt University.

Foreign policy criticism

Bolton also launched an outspoken attack on Trump's foreign policy approach, saying the President's North Korea policy was doomed to fail, and called for the administration to exert more pressure on Iran.

Referring to the administration's North Korea policy, Bolton told the audience that "it was perfectly evident it was going to fail."

"There is not a single piece of evidence that the government of North Korea has made a strategic decision to give up the pursuit of nuclear weapons," he added

On Iran, Bolton made clear he wanted the administration to take further steps.

"I don't think we are applying maximum pressure," Bolton said.

He said the sanctions enacted by the Trump administration have had "a very significant effect" but made clear he would like the US to explicitly push for regime change in the country.

Bolton, whose views of Trump's policies toward Ukraine were cited by multiple impeachment witnesses, hasn't publicly commented about the events that unfolded last year.

He was dismissive about the President's attacks against him.

"He (Trump) tweets, but I can't talk about it. How fair is that?" he said adding, "for now, I'm going to let it go."

Battle over Bolton's book

Last week, Sarah Tinsley, a spokesperson for Bolton, said that he had received a new letter from the NSC addressing the contents of his manuscript, although her statement did not specify whether the NSC would further look to delay the book from publishing. Rather, it pointed to "serious concerns that the process has been thoroughly breached and that it is more about suppressing Ambassador Bolton's book than about classification issues."

According to a person familiar with Bolton's post-departure dispute with the White House, a meeting with Bolton's lawyers and the NSC to go over the content of the book remains a possibility, although the timing of any such meeting isn't clear.

The White House has said that it is reviewing the manuscript, a process that can take months, even in the most straightforward of cases. It is also not unusual for the White House to raise concerns over the sensitivity of details written in manuscripts.

Under Executive Order 13526, the US government uses three levels of classification to designate sensitive information: confidential, secret and top secret. Confidential -- the lowest level -- designates information that if released could damage the country's national security. The other designations refer to information the disclosure of which could cause "serious" -- secret -- or "exceptionally grave" -- top secret -- damage to national security.

The President, Vice President and some agency heads designated by the President, have broad authority over classifying or declassifying information.

Bolton, Trump's third national security adviser, who left the White House in September after disagreements with the President over policies on Afghanistan and North Korea, went through the standard NSC ethics clearance process before his departure and was cleared, this person said.

Protocol requires every White House official, upon their departure, to turn over "every record that they create that has to do with the duties of the office," explained one of the sources.

Bolton, a longtime public servant and copious notetaker, destroyed many of his physical notes from his time as national security adviser, as all officials are encouraged to do while they are on the job, a person close to him said.

It also isn't clear whether the NSC is requesting all hard and soft copies of Bolton's manuscript, something they would typically do if there was genuine concern about the publication of classified or highly sensitive information. The NSC may also confiscate Bolton's hard drive if that were the case, although a person close to Bolton said that has not happened.

Bolton began working on his book after he departed the NSC and did not review the notes he took while he was there for the book, a person close to him said.

Possibility book material may be retroactively classified

Bolton's team is also preparing for the possibility that the administration may try to retroactively classify some of the material in his book -- a measure that is uncommon, but not unheard of, due to loopholes in executive order 13526.

"The President is really empowered by the words in this executive authority," said J. William Leonard, the former director of the information security office which overseas executive branch implementation of the executive order, and former deputy secretary of defense.

"It's an authority that can be abused and it can weaken the classification system," Leonard said. "They abuse it to keep the information out of the hands of adversaries that could potentially use the information to harm the country."

A handful of former Secretary of State Hillary Clinton's emails, at the heart of the controversy over her use of a private server for government information, were retroactively classified, although in that case, virtually all the emails contained sensitive information that were recategorized as classified after they were sent to Clinton's unsecure box.

Lawsuits can also result from government intervention in the publication process, as was the case with a book written by former Defense Secretary James Mattis' Pentagon speechwriter. Retired Navy Cmdr. Guy Snodgrass' book was held up by the Pentagon's prepublication review -- a different office than the one reviewing Bolton's manuscript, but one that follows similar protocol. Snodgrass and his lawyer sued the Pentagon, who finally allowed the book's publication with a few amendments.