In the aftermath of costly U.S. frustrations in Vietnam, Iraq and Afghanistan, advocating foreign policy “realism” is often shorthand for modest assessments of what U.S. power can accomplish in a world less malleable than many policymakers once thought. Those three interventions involved attempts to cause democratic institutions to take root in inhospitable social soil. The U.S. intervention in Ukraine — the proxy war with Russia is a war — is different. And the cost of losing even, perhaps especially, a proxy war can be steep.

When Secretary of State Marco Rubio says we should be “realistic” regarding Ukraine, he is not, one hopes, worrying about U.S. hubris. The bigger danger is excessive pessimism about what can be achieved, and a too-sanguine calculation of a low cost of choosing defeat.

Before he became vice president, JD Vance said, “I don’t really care what happens to Ukraine.” Richard Moore, head of Britain’s foreign intelligence service, MI6, says the cost of not supporting Ukraine “would be infinitely higher” than the cost of supporting it. Elaine McCusker, Frederick W. Kagan and Richard........

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