The commander of US troops in Europe says Nato cannot rapidly deploy large forces to Eastern Europe in the way that Russia can.
Lt-Gen Ben Hodges was speaking to the BBC's Hardtalk programme during large Nato exercises in north-western Poland.
"The Russians are able to move huge formations and lots of equipment a long distance very fast," he said.
Nato needs to have that speed too, he said. "Three days' notification, we ought to be able to do that," he said.