This book is full of examples of people coming together: https://en.wikipedia.org/wiki/Nonzero:_The_Logic_of_Human_De...
To a point. But as jacobush points out in a sibling comment to yours, those aggressors could be local as well as foreign: the myth of the benevolent king protecting the poor from rapacious nobles. Think of Richard II rallying the revolting peasants in the wake of Wat Tyler's death (ultimately to those peasants' detriment, alas). And arguably the main source of a monarch's legitimacy was religious: he (or she) was seen as ordained by God.
> In fact, one of the main functions of a nation-state is to put up defence against other nation-states.
Well, yes. But providing a social safety-net for its most vulnerable people is also one of the main functions of a nation-state: it doesn't thus follow that nation-states formed for that reason.
A nation-state will likely be more able to defend itself than a looser, more heterogeneous polity, but it will also provide a better vehicle for aggression, and that aggression will typically be employed in pursuit of the interests of the powerful. When the Tudors laid the foundations of a nation-state in 16th century England, was it as a defensive measure, or to pursue dynastic interests in France and Scotland?
With a few exceptions (Switzerland comes to mind), the idea that a nation-state represents the conscious union of a people as a whole for defence against outsiders seems more like propaganda and national myth-making than an accurate recounting.
This is the synergystic existense between so-called leaders. They keep each-other in power, both using the threat of the other.