One secret to the North Atlantic Treaty Organization's (NATO) longevity has been its ability to continually adapt to the ever-changing external security environment and needs of its members. This is all the more needed today in a world that is at once more complex and more interconnected. Over the next decade, NATO's ability to adapt and remain relevant for the future will depend on both external and internal drivers of cohesion and division. External drivers include the threats and opportunities facing the alliance and its individual members, the availability...