"It's not hard to imagine Donald Trump leading us into a war just because somebody got under his very thin skin," Clinton said.
"Donald Trump's ideas aren't just different. They are dangerously incoherent. They're not even really ideas — just a series of bizarre rants, personal feuds and outright lies," she said.
She went on to slam him for "praising dictators like Vladimir Putin" and picking fights with friends of the U.S. like the British prime minister, the mayor of London, the president of Mexico and Pope Francis.
"He says he has foreign policy experience because he ran the Miss Universe pageant in Russia," Clinton said, prompting laughs from the audience.
His favorable comments about Putin and North Korean dictator Kim Jong Un came under criticism from Clinton, who said it showed his affinity for "strongmen."
"I will leave it for psychiatrists to explain his affection for tyrants," she said.
Clinton took aim at Trump's tendency to tweet out accusations, and she joked that she was "willing to bet that he's writing a few about me right now."
"Imagine if he had not just his Twitter account at his disposal when he's angry but America's entire arsenal," she said later in the speech.
"Making Donald Trump our commander in chief would be a historic mistake, and it would undo so much of the work that Republicans and Democrats alike have done over many decades to make America stronger and more secure," she said.
Today's speech was her first on national security since Trump became the presumptive Republican presidential nominee in early May.
Both parties are holding their primaries in California on June 7, and a new poll from Field Research Corp. has Clinton beating Trump in the state by 19 points in a general election matchup.