We can posit two types of identity that a group might have: a negative identity, as in not being something else, and a positive identity, comprising how it sees itself through what it strives to be. That positive identity can include continuity with the past but must also address goals, purpose, and defining unique capabilities or traits.
A German may say, for example, “Thank God we are not the verdammt French,” but when looking for a positive identity, might say that Germans produce great order, efficiency, structural elegance, and mental and moral clarity. The latter constitute a positive identity, while the former distances the group from others around it.
Having a negative identity proves to be fairly easy; you simply have to declare that you are not something similar and break away. For example, South Virginia can point to West Virginia and now-North Virginia and say, “Look, we are different here now,” and the distinction itself serves as severance.
Without a positive identity, groups suffer a lack of cohesion. Negative identity allows distinction from others, but without some sense of shared purpose or goal, groups fall apart. This explains why groups can unite in wartime, but then disintegrate shortly thereafter.
We can see this happening in America through the political fragmentation of its Congress, which unites during wartime but then rapidly falls into partisan bickering:
When an external threat looms, even a democracy can unite, but as soon as it disappears, unity fragments. This graph explains among other things why America went far-Left after the Cold War: we had been united merely by the presence of an enemy, but once it fell, the Left went further toward its extreme, in turn driving the Right away in order to distinguish itself.
This shows us a weakness in our current political system. No unifying force exists without an external enemy, so if we were to produce an internal enemy — preferably an abstract one, like Leftism — while producing a contrary vision, it would cause a convergence of political extremes at the same time uniting the group.