Mark Esper said Trump saw US allies like Canadian Prime Minister Justin Trudeau as "weak."
Meanwhile, Trump saw himself and leaders like Russia's Vladimir Putin as "strong."
Trump frequently praised authoritarians like Putin and Xi throughout his presidency.
Former Pentagon chief Mark Esper in his new book wrote that former President Donald Trump saw a number of leaders of US-allied countries as "weak," but viewed US adversaries like Russian President Vladimir Putin as "strong."
"I would quickly learn that 'weak' was the president's favorite word when describing adversaries or opponents, whether in the United States or abroad. It contrasted sharply with 'strong,' which was how he regarded himself, and how he wanted others to see him," Esper wrote in "A Sacred Oath: Memoirs of a Secretary of Defense During Extraordinary Times," which was released on Tuesday.
Esper said that Canadian Prime Minister Justin Trudeau, former German Chancellor Angela Merkel, and former British Prime Minister Theresa May (Merkel and May were in office during Trump's tenure) "were in the 'weak' category" - despite leading leading democracies that are among the US closest allies.
Trump had a "special contempt" for Trudeau, according to Esper.
"Russian President Vladimir Putin, Chinese President Xi Jinping, and Turkish President Recep Tayyip Erdogan were all in the other column," Esper added, referencing the three authoritarians. Though Erdogan is a NATO leader and Turkey has long been a US ally, the Turkish leader's authoritarian tendencies have seen the relationship between Washington and Ankara sour.
After more than two decades of Putin's rule, Russia is an authoritarian system with sham voting processes that are highly controlled by Putin and his underlings. And Xi, as chairman of China's Communist Party, is unelected and preparing for a rare third-term in power.
Throughout his presidency, Trump often insulted US allies like Germany over issues like defense spending and NATO.
Meanwhile, Trump frequently praised US adversaries like Putin and Xi - though his dynamic with the Chinese leader became thornier over the course of 2020 as Trump repeatedly blamed the COVID-19 pandemic on Beijing (after initially lauding Xi's handling of the virus).
Trump entered the White House during a period of historic tensions between Washington and Moscow, linked to issues ranging from the Ukraine and Syrian wars to Russian election interference. Despite the contentious dynamic between the two countries, Trump routinely went out of his way to avoid criticizing Putin and defended him at times.
In one of the most scandalous moments of his presidency, Trump appeared to side with Putin over the US intelligence community on the subject of Russian election interference as the two leaders met in Helsinki, Finland in July 2018. Trump ultimately walked back on his comments in Helsinki, but his tendency to express admiration for Putin continued well after that episode.
During a Fox News interview last April, for example, Trump said, "I got along great with President Putin. I liked him, he liked me. That's a good thing, not a bad thing."
More recently, Trump in February praised Putin's false justification for invading Ukraine as "savvy" and "genius."
Experts on authoritarianism and democracy have said Trump's behavior as president emboldened and enabled authoritarians like Putin.