From
The Nation (New York), an argument in 1870 that rivers can never form "natural" boundaries between nations:
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NATURAL BOUNDARIES
By Michael Heilprin
(September 1, 1870)
When the power of Napoleon I was rapidly crumbling away after the crushing defeat at Leipzig, the allies, halting at Frankfort before entering upon the last campaign, offered him, for peace, the undisturbed possession of France, with her limits extended east to the banks of the Rhine. The France thus offered him would have been almost coextensive with ancient Gaul, which was bounded by the Rhine, the Alps, and the Pyrenees, and would have embraced, besides the French Empire as it now is, the whole of Belgium, portions of the Netherlands, Luxemburg, and Rhenish Prussia, Hesse, and Bavaria. Napoleon, in his unreasonable pride, spurned these terms of peace, and when, a few months later, he presented them as his own to the Peace Conference at Chatillon, they were rejected by the allies. Napoleon fell, and the kingdom of the Bourbons was ultimately reconstructed as it had been before the wars of the Revolution. But since that time France has not ceased dreaming and talking of her natural boundaries — the Pyrenees, the Alps, and the Rhine. And this has not been the idle dream and idle talk of popular vanity and demagogism merely; statesmen, historians, publicists, and poets have vied with each other in making France believe that she had a natural right to all the lands west of the Rhine, and the dire consequence of that fondly cherished delusion is the present war.
We call it a delusion, for the Rhine is not a natural boundary of France in a rational sense of the word. Nor are rivers, in general, the natural boundaries of countries. Rivers, it is true, form excellent geographical lines of demarcation between provinces or other divisions of one and the same empire, kingdom, or confederation, such as are the lines of the Ohio and the Mississippi, which bound some of our non-original States. But they are no more real lines of separation than are the meridians of longitude or parallels of latitude which have been selected to bound other States of our Union. For rivers, especially navigable rivers, far from being separating barriers, are natural channels of intercourse and intermingling, of coalescence and union, the world over. Comparative geography, a science of rather recent development, has fully established this axiom. If used as real barriers, as the Rhine and Danube were by the Romans against the barbarians, and the Ticino and Po by the Austrians against Italy, they form unnatural barriers — that is to say, unnatural boundaries — kept up and guarded by the sword of the conqueror, occasionally long enough to become, or at least to appear, natural. Watersheds, not rivers, form natural boundaries. Mountain ranges separate nationalities. The same nationality almost everywhere flourishes on both banks of every navigable river. Every basin, or at least every section of a basin, has its character. The inhabitants of the slopes that hem it in will fuse with the dwellers in the bottom. People living on the opposite slopes of a mountain range will tend in opposite directions.
The whole of history and geography, studied together, proves it. The Nile has never nourished two different nationalities on its opposite banks; it has never been the boundary of an empire. Babylonia flourished on both sides of the Euphrates; Assyria on both sides of the Tigris. The Hebrews occupied both banks of the Jordan. Neither the Oxus nor the Jaxartes, neither the Indus nor the Ganges, neither the Yang-tse-kiang nor the Hoang-ho, has ever formed a boundary between different nationalities, or separated different civilizations. It was not the river Eurotas, the Alpheus, the Cephissus, or the Peneus, but mountain ranges like the Taygetus, the Pindus, and the Œta, that formed, by bounding, the wonderful system of Grecian autonomies. The various sections and branches of the Apennines mainly separated the ancient national divisions of Italy. Rome developed its power on both banks of the Tiber; the Po, in forming Cispadane and Transpadane Gaul, bounded provinces but separated no nationalities; the little rivulet Rubicon only marked the end of a frontier line formed by the Apennines, just as the little Tweed in the Middle Ages served to complete the natural boundary line of the Cheviot range between England and Scotland.
Mountain ranges, not rivers, formed, in the Middle Ages, the grand divisions of the Iberian Peninsula. The Ebro flows not on the confines but through the midlands of Aragon; the Guadalquivir does not bound but traverses Andalusia; Castilians live on both sides of the upper Douro and Tagus, Portuguese on both sides of the lower. The countries of Eastern and Central Europe show striking parallel examples. Russians inhabit both banks of the Volga and the Don, Poles both banks of the Vistula; Germans both banks of the Oder, the Elbe, the Weser, and the Rhine. The Danube flows through the very centres of Wurttemberg, Bavaria, Austria, and Hungary. The last-named polyglot country owes its national unity mainly to the encircling wall of the Carpathians; all its rivers flow towards or through its central bottom lands, and thus keep up a union even of the most heterogeneous elements. Bohemia is a mountain quadrilateral. . The mountain and river systems of the rest of Europe confirm the rule, with hardly a single exception. Neither do those of America invalidate it. That the Father of Rivers is a mighty bond of union instead of a barrier of separation, is acknowledged on all hands. The same is the case with the Missouri. A glance at the map will show that the St. Lawrence is only a figurative boundary line between the United States and the British Provinces, and that it flows through the latter. The Rio Grande is a frontier line dictated by recent conquest, and Indian tribes continue to roam on both its banks. Rivers selected as State lines are too feeble even as barriers between communities. The lower western bank of the Hudson is lined with suburbs of New York City. Camden is a suburb of Philadelphia; Covington, of Cincinnati. In South America, the Amazon and the Orinoco offer parallel instances to the Mississippi and the St. Lawrence. Some branches of the La Plata alone can be said to form exceptions, but recent events indicate that even these are not to last.
To return to the natural boundary between France and Germany. It is clear that the Rhine is far from forming it, either geographically or historically. The natural geographical boundary line, irrespective of the now existing nationalities, is the watershed between the Meuse and the Aisne and Marne, and its easterly continuation between the head-waters of the Saone and Doubs, on one side, and those of the Moselle and Ill, on the other. All of France that lies east and northeast of this watershed — the main parts of Lorraine and the whole of Alsace — belongs to the water-system of the Rhine, a river both banks of which, from its source to its mouth, are inhabited exclusively by Teutonic people — Swiss, Germans proper, and Dutch. Historically, the lands watered by those western affluents of the Rhine formed, after the downfall of the Roman rule in Gaul, parts of the Frankish realm of Clovis, and subsequently of its eastern and purely German division, Austrasia, while the valleys of the Seine and of its numerous affluents formed the much more Gallic western division, Neustria. The Carlovingian Empire embraced both divisions, but after its final disruption during the period of partitions inaugurated by the Treaty of Verdun, Austrasia was merged in Germany, while out of Neustria gradually grew up the modern Kingdom of France. And both Alsace and Lorraine — the latter in its main parts — continued to belong to Germany down to the time when French centralization, developed by Louis XI and perfected by Richelieu, proved itself decidedly superior to the more and more loosening machinery of the Empire — the final annexation of the two provinces to France taking place under Louis XIV and Louis XV respectively. The inner territories of Lorraine have since become almost entirely Gallicized; Alsace is French in sentiment, though not in language, and the section of the Rhine which bounds it on the east has assumed the semblance of a natural boundary, but the semblance only. The possession of the western bank of this river section has stimulated the desire of making the Rhine the eastern boundary of France. The constant threatening to achieve this conquest as an act based on a natural postulate has awakened, even in the more moderate portions of the German people, the thought of reestablishing, on an opportune occasion, the natural boundaries between Germany and France as they were before the Peace of Westphalia. It is beyond the sphere of this article to discuss the questions whether the present is the opportune moment to do it, and whether it would at any time be just or expedient to do it against the will of the populations concerned.
https://books.google.com/books?id=AzYMAQAAIAAJ&pg=PA254