U.S. Army soldiers move an armored Stryker vehicle into position during live-fire training. Carlo Angerer / NBC News

Reliant on American support and fearful of Russian influence, European leaders will be closely watching

"The stakes are pretty high for Europe in terms of how that meeting turns out," said Susi Dennison, a senior policy fellow at the European Council on Foreign Relations. "For Europe, how the personal meeting goes between these two is going to be pretty crucial."

European leaders are also unsure whose word actually represents U.S. policy, according to Matthew Harries, a research fellow at the International Institute for Strategic Studies, a British research institute.

Establishment figures in his team, such as Defense Secretary James Mattis and National Security Adviser H. R. McMaster, have been far more clear than their boss in supporting NATO's allies across the pond.

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In May, after Trump failed to endorse Article 5 during a speech at NATO's headquarters in Brussels, McMaster, told journalists later that "of course" the president backed the principle of collective defense.

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"Nobody's entirely sure who speaks for the U.S. and whether what the president says is official policy, which is very unusual," said Matthew Harries, a research fellow at the International Institute for Strategic Studies, a British research institute.

"European leaders are perfectly happy with Mattis but their problem is with Trump," Harries added. "Does the president speak for the U.S. or does the defense secretary? If it's Mattis then Europe will be happy. If it's Trump then they won't."

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US Troops Reassure Allies in Poland Ahead of Trump's G-20 Visit - NBCNews.com