David Cameron has been warned that Britain cannot afford any more defense cuts in the wake of the growing threat posed by Vladimir Putin—despite claiming the UK military is strong enough to fend off Russian aggression. The Conservative chair of the Defense Select Committee, Rory Stewart, said Mr. Putin was watching to see if Britain was going soft on defense. He said Britain’s military could not stand another round of defense cuts after the election. Mr. Stewart’s stark warning comes after RAF fighters were scrambled to escort two Russian bombers in international airspace off the coast of Cornwall yesterday. A...