Should the Oregon question lead to hostilities between the United States and England, the theatre of war would embrace the greater part of North America and the two oceans, but the theatre of operations would probably be limited to Canada and our northern frontier, with naval descents upon our maritime cities.
The first point to be attended to in a plan of military operation is to select a good _base_.

Many circumstances influence this selection, such as mountains, rivers, roads, forests, cities, fortifications, military depots, means of subsistence, &c.

If the frontier of a state contain strong natural or artificial barriers, it may serve not only as a good base for offensive operations, but also as an excellent line of defence against invasion.

A single frontier line may, however, be penetrated by the enemy, and in that case a second or third base further in the interior becomes indispensable for a good defence.
A French army carrying on military operations against Germany would make the Rhine its first base; but if driven from this it would form a second base on the Meuse or Moselle, a third on the Seine, and a fourth on the Loire; or, when driven from the first base, it would take others perpendicular to the front of defence, either to the right, on Befort and Besancon, or to the left, on Mezieres and Sedan.

If acting offensively against Prussia and Russia, the Rhine and the Main would form the first base the Elbe and the Oder the second, the Vistula the third, the Nieman the fourth, and the Dwina and the Dnieper the fifth.
A French army operating against Spain would have the Pyrenees for its first base; the line of the Ebro for a second, resting its wings on the gulf of Gascony and the Mediterranean.