Nato’s top military commander said the alliance is “ready to fight tonight” if it is attacked, as European officials warn Russia could test Nato’s defences with increased covert attacks or a limited military incursion. European nations have in recent months faced an escalation of covert Russian activity, including an arson attack on an Estonian defence company and a failed attempt to attack a Ukrainian aircraft at a German airport. If Russia decides to attack Nato, the alliance will respond by...