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. 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. 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 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.