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And "during this administration, it's a little diplomacy, lots of vibes," former State Department spokesperson Ned Price said. "The bar for success at a multilateral summit like this is quite low during this administration," Price added, saying that the bar "really amounts to avoiding a massive blowup either in person or on social media."

Last year's gathering didn't set a strong precedent. Trump abruptly walked out of the confab in Canada a day early, returning to Washington as tensions rose between the U.S., Israel and Iran.

There's a distinct possibility that a repeat performance is in store. Even though the Trump administration has come to a preliminary agreement with Iran, there's no obvious path to a long-term agreement.