So what did Trump mean by that remark?
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stand back
[stand back]
DEFINITION

    withdraw from a situation emotionally in order to view it more objectively.
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stand by
[stand by]
DEFINITION

    be present while something bad is happening but fail to take any action to stop it.
    "he was beaten to the ground as onlookers stood by" · [more]
        be ready to deal or assist with something.
        "two battalions were on their way, and a third was standing by" · [more]
        synonyms:
        wait · be prepared · be in (a state of) readiness · be ready for action · be on full alert · be at battle stations · wait in the wings
        support or remain loyal to someone, typically in a time of need.
        "she had stood by him during his years in prison"
        synonyms:
        remain/be loyal to · stand up for · support · give one's support to · [more]
        adhere to or abide by something promised, stated, or decided.
        "the government must stand by its pledges"
        synonyms:
        abide by · keep (to) · adhere to · hold to · stick to · observe · heed · comply with · act in accordance with

With Trumps high IQ when he speaks some word meanings need to be defined. All these people went off the rails with that comment he said.

"The president clarified, “I don’t know who Proud Boys are, but whoever they are, they have to stand down. Let law enforcement do their work.”