Jim Mattis issued his most direct attack at the president since resigning as Defense secretary and assured there is more to come.

'There is a period in which I owe my silence. It's not eternal. It's not going to be forever,' Mattis told The Atlantic in an article published Thursday.

The retired Marine Corps General who stepped down as Pentagon chief at the end of 2018 said he felt he owed a period of silence to the administration and American people following his departure – especially on the grounds he decided to leave.

'If you leave an administration, you owe some silence. When you leave an administration over clear policy differences, you need to give the people who are still there as much opportunity as possible to defend the country,' Mattis said.

'I know the malevolence some people feel for this country,' he continued, 'and we have to give the people who are protecting us some time to carry out their duties without me adding my criticism to the cacophony that is right now so poisonous.'

Former Defense secretary Jim Mattis said said he owed the administration a period of 'silence' following his resignation, but claimed 'it's not going to be forever'

Mattis resigned after butting heads with President Trump on several policy issues. He has recently begun speaking out against the president and explaining why he decided to quit as Pentagon chief

After serving just short of two years as Trump's Defense secretary, Mattis submitted his resignation letter on December 20, 2018 – just one day after Trump announced he was withdrawing troops from Syria.

Shortly before Trump's announcement, Mattis said troops would remain in Syria following the defeat of ISIL to ensure the terrorist organization did not regroup.

The two also had several different perspectives on policy.

On Wednesday, Mattis explained that he left the administration because Trump was ignoring his advice to keep faith in U.S. allies.

'When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution,' he wrote in the Wall Street Journal op/ed.

Despite being at odds with the president for most of his tenure as Pentagon chief, Mattis said attacking him following his departure would weaken the president at a time when the country is facing real external threats.

'You don't endanger the country by attacking the elected Commander in Chief,' he said in his interview. 'I may not like a Commander in Chief one fricking bit, but our system puts the Commander in Chief there, and to further weaken him when we're up against real threats—I mean, we could be at war on the Korean peninsula, every time they start launching something.'

Mattis said even if he doesn't like the Commander in Chief 'one fricking bit' he wouldn't speak out against him because it could 'further weaken him' as the U.S. faces external threats

Mattis, however, did claim that Trump is being counterproductive with his foreign policy initiatives and international interventions.

North Korea has fired several short-range missiles, which the president claims does not violate the agreement made at the Singapore Summit in 2018 where Trump and North Korean Leader Kim Jong Un held bilateral meetings to work toward denuclearization.

'North Korea fired off some small weapons, which disturbed some of my people, and others, but not me,' Trump tweeted in May. 'I have confidence that Chairman Kim will keep his promise to me, & also smiled when he called Swampman Joe Biden a low IQ individual, & worse. Perhaps that's sending me a signal?'

Mattis wasn't satisfied with the classification, claiming his dismissal of the missiles tests were 'beneath the dignity of his office.

'Any Marine general or any other senior servant of the people of the United States would find that, to use a mild euphemism, counterproductive and beneath the dignity of the presidency,' Mattis said.

The former Defense secretary has been emerging more publicly lately, after laying low following his administration departure. Emerging back to public life comes shortly before the release of his upcoming book Call Sign Chaos: Learning to Lead, which will hit shelves September 3.

Mattis said his resignation letter is included in the book to help people better understand why he decided he could no longer serve in the Trump administration.

'I had no choice but to leave,' he said. 'That's why the letter is in the book. I want people to understand why I couldn't stay. I've been informed by four decades of experience, and I just couldn't connect the dots anymore.'