The latter path is no longer just a meddlesome aspect of an otherwise sound alliance. It threatens a messy separation between the United States and its foremost allies that would hurt European and American security. It is in the United States’ interest that Secretary of State Marco Rubio, who is leading the Trump administration’s contingent at the conference and the accompanying congressional delegations, maximize progress down the first path. And no matter how Mr. Rubio sets the tone, Europe must come together behind meaningful reform.