Ukraine last week got a vital $61 billion lifeline from the United States. But it still needs a medium-term funding plan to withstand Russia’s onslaught. A multi-year funding plan would have several benefits. For a start, it would provide some insurance against U.S. political swings. It would also boost Ukrainian morale and give Western arms manufacturers greater confidence to ramp up production. Ukraine is straining every sinew to hold the line against Russia, an adversary whose $2 trillion, opens new tab economy was 11 times bigger than its own last year. How much Kyiv needs in future depends on what...