The former is rarely compatible with your plan, and necessarily retards its execution; and hence the preference which is generally given to the latter.
2d.

When reinforcements are about to join you, and this can only be effected by a march through a country actually occupied by hostile corps, or liable to be so occupied, you must again waive the general rule, and risk one party for the security of the other; or, (which may be better,) make such movements with your main body as shall accomplish your object.
3d.

When you have complete evidence of the actual, or probable insurrection in your favor, of a town or province of your enemy, or of a division of his army, you must support this inclination by strong _detachments_, or by movements of your main body.

Napoleon's operations in Italy, in 1796-7, furnish examples of what is here meant.
4th.

When, by dispatching a _detachment_, you may be able to intercept a convoy, or reinforcement, coming to the aid of your enemy.
These are apparent rather than real exceptions to the rule of concentration.