Abstract: Modern constitutionalism cannot be well understood without considering the interaction between domestic law and international politics. Taking the first modern written Constitution as an example, this article illustrates the domestic/international intermixture by tracing out the genealogy of America’s geostrategic constitution, that is, the relationship between American constitutionalism and external factors, which has been largely underestimated by existing scholarship. It shows that original constitution-making took place in a great geostrategic crisis, and the framers drafted the Constitution with considerable geographical, national-security considerations. The chief aim of the U.S. Constitution was not to protect individual rights, but rather empower the central government that can responded to external threat. In its subsequent operation, geostrategic factors also played a great role in constitutional interpretation and constitutional transformation, especially through the opinions of Chief Justice Marshall. Considerations on external factors sit at the center in the American experiment of republican self-government. But the cost also deserves attention: geostrategic concerns facilitates the expansion of the executive power.