This is the great genius of Hamilton on full display, and his brilliant theory furnishes a compelling explanation for why the American union has persisted for so long. Our mutual success depends on the union itself, such that our fates are now so intertwined other that it is impossible to separate them. We can be Protestant, Catholic, Jew, Muslim, or atheist. We can be black, white, or Latino. We can be Northern or Southern. We can be liberal or conservative. We can have any number of professions. But so long as we continue to appreciate that our own personal prosperity depends on the prosperity of those with whom we may otherwise disagree, we shall remain together.

This is one reason that the Civil War had an economic component, although the dominant issue was of course slavery. The North and the West had been rapidly expanding and industrializing through the first half of the 19th century, integrating the regional economies. But the South remained a world unto itself, and much of its wealth was generated by exporting cotton to Europe. Indeed, one of the main reasons the South reckoned that it could successfully rebel is that the European powers would miss southern cotton exports so badly that they would broker a settlement.

Nothing like the South of the 1850s exists today. America is well integrated economically. So Hamilton’s old logic is fully in force: Mutual economic gain remains the keystone of the national union.