When President Volodymyr Zelensky joined U.S. President Donald Trump at the White House on Aug. 18, the positive mood between the leaders stood in stark contrast to their disastrous encounter in February in the same spot.

On Sunday, Secretary of State Marco Rubio, who sat in on the meetings with Mr. Putin at the American air base outside of Anchorage, disputed the idea that the Europeans were coming as a posse to protect Mr. Zelensky from a repeat of the February shouting match.

It is quite possible that Monday's meeting in the White House could prove even more crucial to the future of Ukraine - and for all of Europe's security - than last Friday's US-Russia summit in Alaska. On the surface, that Putin-Trump reunion seemed to live down to every expectation.