On Monday the White House released a new national security strategy that named Russia as a “revisionist” power seeking to upend global order. On Wednesday President Trump followed up by confirming he’d approved the sale of lethal defensive weapons to Ukraine as it resists Vladimir Putin’s aggression.

Mr. Putin invaded and then illegally annexed Crimea in 2014, and Ukrainians have been fighting since to stop the march of Moscow-backed separatists in their south and east. They’ve had little material support from the West. That’s...