3d. To cover our great cities against a maritime attack and bombardment.
4th. To cover our ship-yards and great naval dépôts.
5th. To prevent, as much as possible, the great avenues of interior navigation from being blockaded by naval means at their entrance into the ocean.
6th. To give to our navy facilities for protecting our coast trade from the enemy's ships of war, and our internal communications, which lie near the coast, from maritime descents.
Let us notice how France has attempted to accomplish this object. The Mediterranean frontier has Fort Quarré, Fort St. Marguérite, St. Tropez, Brigançon, the forts of Point Man, of l'Ertissac, and of Langoustier, Toulon, St. Nicholas, Castle of If, Marseilles, Tour de Boue, Aigues-Montes, Fort St. Louis, Fort Brescou, Narbonne, Château de Salces, Perpignan, Collioure, Fort St. Elme, and Port Vendre. Toulon is the great naval dépôt for this frontier, and Marseilles the great commercial port. Both are well secured by strong fortifications. The Atlantic frontier has Bayonne; the forts of Royan, Grave, Medoc, Paté, &c., on the Gironde; Rochefort, with the forts of Chapus, Lapin, Aix, Oleron, &c., to cover the roadstead; La Rochelle, with the forts of the Isle of Ré; Sables, with the forts of St. Nicholas, and Des Moulines, Isle Dieu, Belle Isle, Fort du Pilier, Mindin, Ville Martin; Quiberon, with Fort Penthièvre; L'Orient, with its harbor defences; Fort Cigogne; Brest, with its harbor defences; St. Malo, with Forts Cézembre, La Canchée, L'Anse du Verger, and Des Rimains; Cherbourg, with its defensive forts and batteries; Havre, Dieppe, Boulogne, Calais, and Dunkirk. Cherbourg, Brest, and Rochefort, are great naval dépôts; and Havre, Nantes, and Bordeaux, the principal commercial ports. Many of the works above enumerated are small in extent and antiquated in their construction, and some of them quite old and dilapidated nevertheless, they have heretofore been found sufficient for the defence of the naval dépôts and commercial seaports of France against the superior naval forces of her neighbor.
Omitting for the present all discussion of sea-coast defences, let us examine more particularly the character and influence of fortifications on land frontiers.
All military writers agree that fortifications have heretofore exerted a great, and frequently a decisive, influence on the operations of a war. Those of France are frequently referred to as proofs of this influence. But, while all are disposed to allow that these works contributed much in former times to the defence of states, yet some have said that modern improvements in the mode of attack have rendered forts far less valuable than formerly.
Such, however, is not the case. Improvements in the mode of attack have not kept pace with the facilities of locomotion; and, although fortifications do not now usually sustain a siege of as many days as in former times, still, as compared with the relative lengths of campaigns in ancient and modern wars, the proportional length of sieges is now even greater than formerly. When the same is accomplished in a campaign of seven weeks as was formerly done in a war of seven years, it is not necessary that fortified places should hold out a very long time. A place that can sustain a siege of a month is now deemed sufficiently strong for ordinary campaigns; for by the end of that time the defensive army will either be destroyed, or be able to come to its succor. In either case a longer defence would not be required.
A reference to the most important sieges of the last century or two will show that forts are, on an average, capable of sustaining a siege for more than that length of time. Lille, in 1708, held the allies in check for a whole year; and again, in 1792, compelled the Austrians to raise the siege after an unsuccessful attack of fifteen days.
Antwerp, in 1585, sustained a siege of fourteen months against greatly superior forces; in 1814 Carnot defended the citadel of this place for four months, and until an armistice had been concluded between the contending parties; in 1832, it sustained, with a garrison of only 4,500 men and 145 pieces of ordnance, a siege of twenty-five days, against a force of 55,000 men and 223 cannon.