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“He was not prepared and Israel was not prepared,” the 2024 Republican front-runner said in an interview with Fox News on Wednesday night.
Mr Trump's remarks are a departure from the overwhelming support US leaders have displayed towards Israel since the country was attacked less than a week ago.
More than 1,200 Israelis were killed in the attacks, while the death toll among Palestinians has surpassed 1,400. Thousands more have been wounded. Children and the elderly are among the dead.
At least 27 Americans have been killed in the attacks. Another 14 are unaccounted for, although it has been confirmed that US citizens are among those being held hostage in Gaza.
In a rally later held in Florida, Mr Trump again lashed out against Mr Netanyahu, accusing the Israeli leader of abandoning him right before the US launched a strike against Iranian Gen Qassem Suleimani in 2020.
“Israel was going to do this with us, and it was being planned and working on it for months,” he said of the plot to kill the leader of Iran's Quds Force.
“The night before it happened, I got a call that Israel will not be participating in this attack.
“I'll never forget that Bibi Netanyahu let us down. That was a very terrible thing.”
“So when I see sometimes the intelligence you talk about … or you're talking about some of the things that went wrong over the last week, they've got to straighten it out because they're fighting potentially a very big force.”
He also called Hezbollah “very smart”.
The White House condemned Mr Trump's remarks as "dangerous and unhinged".
"It's completely lost on us why any American would ever praise an Iran-backed terrorist organisation as 'smart'. Or have any objection to the US warning terrorists not to attack Israel," spokesperson Andrew Bates said in a statement.
Florida Governor Ron DeSantis, who is also running for the Republican presidential nomination, was quick to condemn his rival's remarks.
“It is absurd that anyone, much less someone running for president, would choose now to attack our friend and ally, Israel, much less praise Hezbollah terrorists as 'very smart,'” he said on X.
Mr Trump suggested that the Israeli military is also fighting Iran, although there is no current evidence that Tehran was directly involved in the planning of the attack.
He and other Republicans have also tried to blame President Joe Biden's administration for unfreezing $6 billion in frozen assets to Iran, which is a supporter of Hamas. The White House has said that the money was for humanitarian purposes.
And in a closed-door meeting on Thursday, deputy treasury secretary Wally Adeyamo told Democratic members of Congress that the $6 billion remains unspent and is not going anywhere yet, US media reported.
The White House has also acknowledged that Iran had complicity in the attacks.