By hosting an unprecedented short-notice summit with Ukraine's President Volodymyr Zelenskyy and key European leaders on Monday, President Donald Trump significantly raised the prospects for ending Russia's three-and-a-half-year-long war against Ukraine.

The vibe at the opening was affable and positive. The participants genuinely looked determined to work out compromises that only a few weeks ago appeared illusory. It was a good sign for long-term Euro-Atlantic security cooperation in the face of challenges that, in Trump's words, we have not faced since World War II. Toward the end, Trump's call to Moscow brought a follow-up U.S.-Ukraine-Russia summit within reach.

But the rising expectations also reveal formidable obstacles on the path to peace. As the world's leaders were heading to W

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