WASHINGTON—President Donald Trump said a suspected chemical attack by the Assad regime was “a terrible affront to humanity” that changed his mind about the Syrian strongman, signaling a more aggressive U.S. policy toward Syria.

Mr. Trump didn’t elaborate on how his administration would respond to the latest attack, which killed at least 85 people, but said it made him re-evaluate his approach to Syrian President Bashar al-Assad and his regime.

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