Gancia, Ponzetto, Ventura provide a precis to their very interesting theory about the size and number of nations.

Before 1950, more than one third of all territorial disputes were decided by war, while after that date diplomacy prevailed in almost 90% of cases.

Why did the first wave of globalisation lead to political concentration and conflict? Why did the second wave of globalisation lead instead to political fragmentation, resolved in a more peaceful way? To answer these questions, in a new paper we develop a model to study the interaction between globalisation and political structure (Gancia et al. 2017). A key premise of our theory is that borders hamper trade and globalisation make borders more costly. We show that political structure adapts to expanding trade opportunities in a non-monotonic way. In early stages, borders are removed by increasing the size of countries. In later stages, the cost of borders is removed by creating economic unions, and this leads to a reduction in the size of countries. Moreover, while the incentive to conquer markets through aggression increases with globalisation, international economic unions remove this incentive, thereby paving the way to the rule of diplomacy.