NATO Secretary-General Jens Stoltenberg on Monday said the military alliance is poised to increase the number of its high-readiness forces to well over 300,000 as part of what he described as “the biggest overhaul of collective defense and deterrence since the Cold War.” His comments come as NATO leaders prepare to convene in Madrid, Spain this week to decide on the so-called Strategic Concept, the 30-member alliance’s most important document. The text, which is updated roughly every decade, will reaffirm NATO’s values, provide a collective assessment of the security challenges and act as a guide to the group’s future political...