How to Benchmark Victory in Ukraine

When President George W. Bush gave his “mission accomplished” speech on board the aircraft carrier USS Abraham Lincoln only six weeks into the Iraq War in 2003, it quickly became a cautionary tale against declaring victory in an unpredictable war. Washington didn’t withdraw most of its forces until eight years later, and the pullout resembled defeat much more than victory.

Twenty years later in Ukraine, the risk is not declaring victory prematurely—but not defining victory at all.

For more than a year now, Ukraine’s Western supporters have provided it with the weapons,...