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. Without these forces, the relation of men to the land
would have tended to modify everywhere in the same way. Increase in