President Donald Trump's substitution of the slightly different terms during his highly anticipated speech in Saudi Arabia on Sunday might go unnoticed by the average US listener.

But the subtle change -- or slip, as the White House called it -- could mean the difference between offending Middle Eastern allies and not, a concern for any president looking to create a good first impression with a key ally on a first trip abroad.

Using the word "Islamic," a reference to the religion, in the same breath as "terrorism" could be seen by Muslims as an affront to their faith and actually play into the terrorists' "clash of civilizations" narrative -- reasons why President Barack Obama assiduously avoided the combination during his presidency.

"Islamist," meanwhile, refers to political movements that seek to implement Islamic law and theology, making it less objectionable to Muslims when paired with "terrorism," the idea goes.

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Trump, however, often repeated the more controversial "radical Islamic terrorism" on the campaign trail in a deliberate and implicit criticism of Obama, implying that his preferred phrasing betrayed naivete or, worse, political correctness at the expense of realism about how to defeat the terrorist threat.

Trump's speech marked a departure from his rhetoric on the campaign and in the Oval Office -- at least, to a degree. Although the prepared remarks referred to "Islamist extremism," Trump actually referred to both "Islamists" and "Islamic extremism," as well as "Islamic terror," in the speech.

"There is still much work to be done," Trump said. "That means honestly confronting the crisis of Islamic extremism and the Islamists and Islamic terror of all kinds."

On Sunday night, a senior White House official said Trump's decision to say "Islamic extremism" instead of "Islamist extremism" as written in his prepared remarks was not intentional but the product of exhaustion brought on by the rigorous travel schedule.

"Just an exhausted guy," the senior White House official said.

According to the White House, the 70-year-old President hadn't gotten much sleep. He edited the speech with his aides during the 14-hour journey from Washington, White House press secretary Sean Spicer said, and spent the remainder of the flight reading newspapers, an aide said.

Asked on Air Force One about the President's fatigue, Secretary of State Rex Tillerson told reporters Monday, "He's doing better than I am. And he's got a few years on me."

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Reset with Muslim world

Trump's trip, in which he also is visiting Israel and Europe, gives him an opportunity to reset relations with the Muslim world. The President chose Saudi Arabia -- home to the world's two holiest Muslim sites -- for his first overseas stop in what his top aides have described as an overtly symbolic gesture.

During his presidential campaign, Trump's rhetoric often blurred the lines between Islam at large and "radical Islam" -- like when he said "Islam hates us" in a CNN interview in March 2016. Beyond his Muslim ban proposal and call to surveil US mosques, Trump also said he was open to creating a database of Muslims in the US -- none of which he has disavowed.

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And Trump's outreach to the Muslim world comes as he is pursuing a policy that many have labeled as inherently anti-Muslim. As Trump speaks in Saudi Arabia, his administration is continuing to fight in court to reinstate the travel ban targeting several Muslim-majority countries that Trump hoisted as a key component of his counterterrorism efforts.

Trump's speech on Islam was authored by Stephen Miller, who also wrote the President's travel ban, a White House official has told CNN

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The official said the speech has been put together through a collaborative process inside the White House, but that Stephen Miller was the primary author.

'Tremendous stamina'

Trump's fatigue also invokes comments he made as a candidate: his regular evisceration of "low-energy" Jeb Bush and Hillary Clinton.

"She doesn't have the look. She doesn't have the stamina. I said she doesn't have the stamina. And I don't believe she does have the stamina. To be president of this country, you need tremendous stamina," Trump said during the first presidential debate.