Story highlights "When it comes to credibility problems, though, Donald Trump is in a league of his own," Gates wrote

He also criticized Hillary Clinton on her handling of foreign policy

Washington (CNN) Robert Gates criticized both Hillary Clinton and Donald Trump on their preparedness to handle US foreign policy -- but the former defense secretary reserved his harshest language for the Republican nominee, who he said is "beyond repair."

"When it comes to credibility problems, though, Donald Trump is in a league of his own," Gates, a Republican who led the Defense Department under Presidents George W. Bush and Barack Obama, wrote in a Wall Street Journal op-ed Friday, adding that electing Trump is "too great a risk for America."

"The world we confront is too perilous and too complex to have as president a man who believes he, and he alone, has all the answers and has no need to listen to anyone," Gates wrote. "A thin-skinned, temperamental, shoot-from-the-hip and lip, uninformed commander in chief is too great a risk for America."

Gates called Trump too "willfully ignorant about the rest of the world" to be commander in chief. He specifically criticized Trump's plan to build a wall on the US border with Mexico, his call to torture terrorists and kill their family members, and his praise of Russian President Vladmir Putin.

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