NATO allies have promised Volodymyr Zelensky, the Ukrainian president, dozens of tanks. But when those tanks will actually be deployed — and whether they will arrive in time to repel a looming Russian offensive — is still unclear. In a Sunday night address, Zelensky called for their delivery to be expedited. “We have to make time our weapon,” he said. Fourteen Challenger 2 tanks promised by the British government could be in the country as soon as late March, but the soldiers who will use them have to be trained first. Ukrainian troops arrived in Britain yesterday to begin learning how to operate the tanks, as well as how to use them to fight as a unit. Some of the German-made Leopard 2s pledged by Germany and other European allies could arrive around the same time — but they may not all arrive right away. The speed at which they do will depend on how many tanks are immediately available and how many will need to be refurbished and reprogrammed. American-made M1A2 Abrams tanks are m...