At the height of the North Korean nuclear crisis in 2017, then-U.S. defense chief Jim Mattis feared Washington and Pyongyang were spiraling toward a nuclear war that would “incinerate a couple million people,” according to a new book by journalist Bob Woodward that suggests the two countries were closer to conflict at the time than previously thought.

“Rage,” released last week, reveals scores of instances during the year that highlight the unease felt by many in the highest echelons of the U.S. government, as President Donald Trump responded to North Korean leader Kim Jong Un’s repeated missile launches with personal barbs and fiery threats.

In one example, Woodward writes about how “for the first year of the Trump presidency, Mattis had been living on permanent alert” and that by late 2017, he was “growing increasingly alarmed about the possibility of a war that could kill millions.”