How a Russian invasion of Ukraine would reverberate around the world

"There is going to have to be a response all along that NATO front line that acts as a deterrent ... and you have to have a whole war-fighting strategy around that," said Neil Melvin, director of international security studies at the Royal United Services Institute (RUSI). "In Europe, this would change things enormously -- because we're so far from thinking in those terms," he added. Melvin predicted that nations would require "large enough forces to fight for a long period, to bring in new forces from the US, [and] to fight cyber dimensions." "It's going to be a huge shift."