Nevertheless, the opposite or separating quality of the sea is undoubtedly the more striking and familiar aspect. I have already alluded to its influence upon the relations of Great Britain and France, and of Great Britain and Ireland. Only when England ceased to be a Continental power did the national spirit blossom into any fullness. The project of a Channel Tunnel between the coast of England and France has twice perished because of the invincible and legitimate repugnance of the Englishman to sacrifice his maritime Frontier for no tangible return. It was because of the interposition of the sea that England lost America; that the Dutch and Portuguese lost the greater part of their Indian Empires; that Napoleon, equally with Rome, experienced so many difficulties in Egypt; that the Mexican adventure of France and Austria ended in fiasco; that Spain was robbed almost in a day of her possessions in Cuba, Porto Rico, and the Philippines. In these circumstances the continued connexion of the parent State with her transmarine possessions - which might seem to be an impossible aspiration - is only maintained in modern times by the grant of some form of self-government with a view to attaching the inhabitants of the colony or possession to the seat of imperial authority.
Second in the list of Natural Frontiers may be placed deserts, until modern times a barrier even more impassable than the sea. Asia and Africa afford the best known instances of this phenomenon. For centuries China has been protected by the great Gobi Desert on her north-west border; Samarkand and Bokhara were shielded by the sandhills of the Kara Kum; the Indus valley was severed from the rest of India in its southerly portions by the Sind Desert, while Beluchistan is still safeguarded by the waste that stretches from Nushki to Seistan. Syria found a natural Frontier on the east in the desert that bears her name. Indeed, the whole of western Asia, that part, in fact, which was exposed to Hellenic influences was for centuries cut off from India by the broad wastes of Persia and Turkestan. Egypt, protected on the west by the impassable barrier of the Libyan Desert, and on the east by the desert of the Sinaitic peninsula, has retained a physical identity almost unequalled in history. It was of her eastern desert Frontier that the greatest captain of modern times, who had himself crossed it at the head of an army, wrote as follows in his Commentaries:
Generals who have marched from Egypt to Syria or from Syria to Egypt have in all periods of history considered this desert the greater obstacle the larger the number of horses they took with them. The ancient historians declare that when Cambyses wished to penetrate into Egypt he made an alliance with an Arab king, who caused a canal to flow with water in the desert, which evidently means that he covered it with camels bearing water. Alexander sought to please the Jews so that they might help him in the passage of the desert. This obstacle, however, was not so great in ancient times as it is today since towns and villages existed, and the industry of man contended with success against the difficulties. Today scarcely anything remains between Salihiyeh and Gaza. An army must, therefore, cross the desert successively by forming establishments and magazines at Salihiyeh, Katieh, and El Arish. If this army starts from Syria it must first of all form a large magazine at El Arish, and then carry it forward to Katieh. But these operations are slow, and they give an enemy time to make his preparations for defense.... An army defending Egypt can either assemble at El Arish to oppose the investment of this place, or at Katieh to raise the siege of El Arish, or at Salihiyeh: all these alternatives offer advantages. Of all obstacles which may cover the frontiers of empires, a desert like this is incontestably the greatest. Mountains like the Alps take second rank, and rivers the third. If there is so much difficulty in carrying the food of an army that complete success is rarely obtained, this difficulty becomes twenty times greater when it is necessary to carry water, forage, and fuel, three things which are weighty, difficult to carry, and usually found by armies upon the ground they occupy.
Finally, Africa furnishes the crowning illustration of the Great Sahara, which for centuries not only cut off the Mediterranean belt from the rest of Africa, but cut off the entire remainder of Africa from the civilized world. It was not till the voyages of Prince Henry the Navigator that this long period of isolation came to an end.
In quite recent days, however, deserts as Frontiers have lost the greater part both of their terror and their strength. Like the loftiest mountains, like the stormiest oceans, they have yielded to the all-conquering influence of steam. When Skobeleff advanced to the extirpation of the Tekke Turkomans at Geok Tepe in 1881, he laid a light railway behind him. As the English troops moved up the Nile to the recovery of Khartoum in 1898, the steel rails kept pace with their advance. It was to the desert line, and not to the actual collision at Omdurman, that the Khalifa owed his destruction. The Turkish railways to Syria, if protracted, would soon deprive Egypt of the security on her eastern border which Napoleon described. Given a level desert with a sound foundation, a railroad becomes the easiest of constructions, and built by competent engineers, can be pushed forward in war time at the rate of three miles a day [3]. Similarly it is the aid of railways that has enabled the United States of America to make so light of the great deserts that stretch eastwards from the Rocky Mountains. There are few desert Frontiers in the world that now remain intact. If occasion arose, the doom of the survivors would probably be sealed. It is a question, not of mechanics, but of water and fuel. On the other hand, a ghost of the idea may be said to survive in the still existing preference of uninhabited or thinly peopled tracts as Frontiers over areas of crowded occupation.
I turn to the consideration of the third type of Natural Frontier, namely, mountains. We have already seen that mountains were the earliest of the barriers accepted by wandering man. Prima facie, also, they are the most durable and the most imposing. They are liable to little change (except such as may be indirectly effected by human agency in the shape of roads, railroads, and tunnels), and they are capable of instant and easy recognition. Such has been the position, and the decisive influence of the great mountain barriers of the world - of the Hindu Kush and Himalayas in Asia [4], of the immense and serried ridges that separate Burma from China, of the Caucasus between Asia and Europe, of the Taurus in Asia Minor, of the Alps and Pyrenees (and in a lesser degree the Balkans and Carpathians) in Europe. From the military point of view the labour in crossing a mountain range is commonly great, particularly for armies, and is exposed to many dangers. What it was in ancient times, a century ago, and in more modern days, may be compared in the crossing of the Alps by Hannibal, and again by Napoleon, and in the experience of the Russians in the Balkans. We have yet to see a campaign in which the appliances of modern science in respect of railroads, tunnels, and telegraphs, are seized by one party so as practically to annihilate the mountain barrier upon which the other relies for protection, although the Germans seized and utilized the French tunnel through the Vosges at the commencement of the Franco-German War, and the Boers possessed themselves of the Drakensberg range and the Laing's Nek tunnel in order to facilitate their descent upon Natal.
On the other hand the theoretical superiority of a mountain Frontier may be qualified by a number of considerations arising from its physical structure. Of course a range or ridge with a sharply defined crest is the best of all. But sometimes the mountain-barrier may be, not a ridge or even a range, but a tumbled mass of peaks and gorges, covering a zone many miles in width (for instance, the breadth of the Himalayas north of Kashmir is little short of 200 miles), and within this area the inhabitants may be independent or hostile. Such has been the case with a large portion of the Pathan Frontier of India, where the physical conformation of the border lends an immense advantage to the holders of the mountains against the occupants of the plains. The desire to counteract this advantage and to transfer it to the Cis-border Power has led to the pursuit of what is known as the Scientific Frontier, i.e. a Frontier which unites natural and strategical strength, and by placing both the entrance and the exit of the passes in the hands of the defending Power, compels the enemy to conquer the approach before he can use the passage. It is this policy that has carried the Indian outposts to Lundi Khana, to Quetta, and to Chaman, all of them beyond the passes, whose outer extremities they guard.
In every mountain border, where the entire mountainous belt does not fall under the control of a single Power, the crest or water-divide is the best and fairest line of division; for it is not exposed to physical change, it is always capable of identification, and no instruments are required to fix it. But it is not without its possible drawbacks, of which the most familiar is the well-known geographical fact that in the greatest mountain systems of the world, for instance, the Himalayas and the Andes, the water-divide is not identical with the highest crest, but is beyond it and at a lower elevation. Another parent of much controversy is the ambiguous phrase 'foot of the hills', the different interpretations of which in fixing the Indian Frontier almost produced a rupture between the Indian Government and the late Amir of Afghanistan.
We now come to the important category of Rivers. As the creation of nature, in contradistinction to the creation of man, no Frontiers are more natural. But in another sense, namely, that which is in accord with the natural habits of man, rivers are not natural divisions, because people of the same race are apt to reside on both banks. Thus the Germans are found living on both banks of the Upper Danube, and the Slavs on both banks of the Lower Danube; the Turks or Tartars live on both banks of the Oxus, though to the north they are in Russian (i.e. Bokharan) territory and to the south under Afghanistan; the Indus was not a natural frontier to the Punjab, because Indian peoples, as distinct from Pathans or border men, inhabit the further as well as the nearer bank of the river. So many of the peoples of Laos lived astride the Mekong that the French soon found it to be an impracticable Frontier. The long contest between America, France, and Great Britain over the Mississippi valley, arising out of the French policy of shutting in the United States behind the Alleghanies, and using the Indians as a barrier between the mountains and the river, abundantly illustrated the futility of a river as a permanent boundary. As soon as the Americans broke through the Alleghanies with railroads there was no stopping them by the Mississippi or any other natural Frontier until they reached the sea. In fact the teaching of history is that rivers connect rather than separate. Strategical reasons have almost invariably been responsible for their conversion into Frontiers. As States developed and considerable armies were required for their defence, the military value of rivers, in delaying an enemy, and in concentrating defensive action at certain bridges, or fords, or posts, became apparent, and in the demarcation of larger kingdoms and States, they provided a convenient line of division, everywhere recognizable, and easily capable of defence. It was for this reason that Augustus selected rivers - the Rhine and the Danube - as the Frontiers of the Roman Empire, though strategical considerations soon tempted the Romans beyond, as the English have been tempted across the Indus, and the French by other causes across the Mekong.
Accordingly the advantages and disadvantages of rivers as Frontiers may be thus stated. The position of the river is unmistakable, no survey is required to identify or describe it and the crossing-places frequently admit of fortification. Rivers are lines of division as a rule very familiar to both parties, and are easily transferred to a treaty or traced on a map. On the other hand, they may be attended by serious drawbacks, confronting diplomatists and jurists with intricate problems. Rivers are liable to shift their courses, particularly in tropical countries. The vagaries of the Helmund in Seistan, where it is the boundary between Persia and Afghanistan, have led to two Boundary Commissions in thirty years. The precise channel which contains the Frontier line, the division of islands, very likely new accretions, in the river-bed, the determination of drinking rights or of water-rights in cases where cultivation is only effected by means of irrigation from the Frontier river, the exact identity of the source of a river, if this be mentioned in a Treaty or Convention, or of its main affluent, or, in a deltaic region, of its mouth, the provision required for navigation, police, and fiscal control - all of these suggest possible difficulties in the acceptance of a river boundary, particularly in new or tropical countries, which cannot be ignored. In ancient and civilized States the procedure to be followed in many of these cases is regulated by international agreement or by the Law of Nations. The general principles regarding the navigation of rivers traversing different States were indeed embodied in Articles I08-116 of the Final Act of the Congress of Vienna [5], and have been applied by subsequent Agreements to some of the principal rivers of Europe, Africa, and America.
The last Natural Frontier to which I need here refer is the wellnigh obsolete barrier created by forests and marshes and swamps. The various Saxon kingdoms of England were, for the most part, thus severed from each other. When Caesar first landed in Britain, the head quarters of Cassivelaunus, the British leader, were placed at Verulamium, near St. Albans, which was surrounded by forests and swamps. Arminius gained his famous victory over the Roman legions by entangling them in the forests and morasses of Westphalia, and there is no more poignant picture in history than the description of Tacitus of those mournful scenes [6]. Venice was many times saved from absorption both by foreign invaders, such as Goths and Huns, and by jealous neighbours by her cincture of lagoons. These, however, as cultivation, settlement, and drainage have advanced, are disappearing types of Frontier, of which no more need now be said.