These encourage the development of natural resources and of commerce, and hence lay the foundation for an increased population, by multiplying the relations between land and people.
[Sidenote: Territorial expansion of the state.] A like object is attained by territorial expansion, which often follows in the wake of commercial expansion.

This strengthens the nation positively by enlarging its geographic base, and negatively by forcing back the boundaries of its neighbors.

The expansion of the Thirteen Colonies from the Atlantic slope to the Mississippi River and the Great Lakes by the treaty concluding the Revolution was a strong guarantee of the survival of the young Republic against future aggressions either of England or Spain, though it exchanged the scientific or protecting boundary of the Appalachian Mountains for the unscientific and exposed boundary of a river.

The expansion to the Rocky Mountains by the Louisiana purchase not only gave wider play to national energies, stimulated natural increase of population, and attracted immigration, but it eliminated a dangerous neighbor in the French, and placed a wide buffer of untenanted land between the United States and the petty aggressions of the Spanish in Mexico.

Rome's expansion into the valley of the Po, as later into Trans-Alpine Gaul and Germany, had for its purpose the protection of the peninsula against barbarian inroads.
Japan's recent aggression against the Russians in the Far East was actuated by the realization that she had to expand into Korea at the cost of Muscovite ascendency, or contract later at the cost of her own independence.
[Sidenote: Checks to population.] If a state lacks the energy and national purpose, like Italy, or the possibility, like Switzerland, for territorial expansion, and accepts its boundaries as final, the natural increase of population upon a fixed area produces an increased density, unless certain social forces counteract it.