Let us suppose that, for a frontier of moderate extent, there are _five_ passes, or avenues through which the enemy may approach the interior.

To effectually defend these approaches against the invading army will require, for each, an army of ten thousand men.

Not being able to decide positively on the plans of the enemy, all these communications must be defended at the same time.

This requires a defending army of fifty thousand men.

Let us now suppose each of these passes to be fortified in such a way, that one thousand men will be able to hold the enemy in check, and force him to resort to the operations of a siege; or, at least, to retard his advance till an active army can be organized in the interior, and prepared to meet him in the field.