A Ukrainian artillery crew member carries an artillery shell to a firing position in the Zaporizhzhia region in southeastern Ukraine on Dec. 17, 2023. Ukrainian troops along most of the 600-mile front line remain optimistic, but physically exhausted, with Russian forces on the offensive. (Mauricio Lima/The New York Times) America’s support for Ukraine in its fight against the Russian invasion is in our critical national interest. It mirrors the challenge that our nation confronted in 1940 when President Franklin D. Roosevelt sought to support Great Britain with arms needed to fight Nazi Germany forces that were attacking Belgium, the Netherlands,...