Journalist Bob Woodward, author of the explosive book "Fear: Trump in the White House," told MSNBC's "Morning Joe" on Thursday that the significance of Defense Secretary James Mattis' departure "cannot be overstated."

"It is a sad moment for the country, and for Trump, that Mattis is gone," Woodward, of Watergate fame, told "Morning Joe." "This is somebody who had the experience, had the clout, at least for a while, with Trump."

Mentioning the disagreements Trump and Mattis had regarding NATO, Woodward added that "in a way, for Trump, these things are about cash flow."

"These relationships are not about cash flow, but Trump brings that mentality from New York real estate, and you know, this is one of the other boxes we're in," said Woodward, associate editor at the Washington Post.

The president announced Sunday that Mattis will leave his position on New Year's Day, two months earlier than Mattis said he would leave in his resignation letter last week. That letter was critical of Trump, which may have led to his expedited exit.

"It was Mattis who took Trump over to the Pentagon his first year, and tried to educate him and say, 'Look, we have these security agreements, we have the trade agreements, we have the top secret intelligence partnerships. This works. Yes, you want to change some things,' but Trump sat there and just said 'it’s all BS,' and got angry," Woodward said. "And so what you have here is the non-functioning of the national security apparatus in a way that couldn’t be more dangerous, quite frankly."

A main theme of Woodward's book was — no matter how much advisers like Mattis tried to convince him otherwise — Trump's main foreign policy and national security concern was the U.S. bottom line. In one particularly noteworthy exchange between the two men, Woodward reported that during a debate in which Trump questioned why the U.S. was spending money and committing resources to defend an array of smaller nations, Mattis said "we're doing this in order to prevent World War III."