The U.S.-led war in Iran will dominate Trump's G7 trip to France

Constanze Stelzenmüller, a transatlantic security expert at the Brookings Institution, said leaders will carry that lesson into any meetings with Trump at the summit.

"Any meeting that includes the president raises the prospect of significant uncertainty," she said. "The president's volatility is legendary. He could get upset very quickly. He can be charming at other times. And he can whiplash between one or the other in the blink of an eye."

Yet some analysts say the friction in the relationship has had some unintended consequence: strengthening European cohesion.

Repeated U.S. pressure over trade, defense spending, and security policy has pushed European governments closer together and led to greater strategic autonomy, said Nathalie Tocci, a former top EU foreign policy adviser. She said repeated U.S. threats - many of which were acted on - forced Europe to adapt.

"The Europeans are in a much better place now than they were a year ago," said Tocci who is now a professor of practice at Johns Hopkins SAIS Europe. "I would say there is less bending of the knee going on and there's more willingness to politely sort of be firm on certain issues."

Yet Europe is still in a bind because of Trump's war and the economic fall out.

The United Kingdom is working with France to build a coalition of nations to help with the demining of the Strait of Hormuz once a peace deal is reached. Trump has also said that he expects European and other western countries who depend on the strait to have a role - something officials say will also be discussed at the upcoming summit.

But Trump has other priorities he wants to focus on. At the top of the pile are strengthening economic ties, investment partnerships and the critical mineral supply chain as well as other global pressing issues such as innovation, AI and addressing the Ebola outbreak.

Still, the divide comes at a moment of heightened global risk, which observers such as Bruen - who will be meeting with G7 officials at the summit - say have shown the limits of an American First approach to a global crisis - one that is more isolated and less coordinated with allies.

"If the United States can't contain the fallout from a military operation of our choice against a single country, that at best is a middling power," he said. "How on earth are we going to be able to push back against a larger power, a nuclear power."