Now, the nodal point is of such importance that it merits particular attention. The nodal point, especially if it is established by a bridge, has two great functions in history. It determines the strategy of campaigns (and alters even the tactics of actions), it determines the growth of towns. It has been said that London was made by its bridge. Whether there was a settlement (there probably was) upon the gravelly hill which approached the river from the north, before any bridge was thrown across the tidal Thames, we do not know; but it is certain that the throwing of this bridge gave London its opportunity for development, and what is true of London is true of Paris, of Rouen, of Maestricht, of Cologne, and of twenty other great urban centres in our civilization. Strategically, a commander holding a nodal point retains the opportunity of moving along any one of many lines of movement, and at the same time denies the opportunity of junction to his enemies. To put it in its simplest form, a commander holding a nodal point and concentrated there can prevent the concentration of two fractions of his enemy along any two roads radiating from that nodal point. He can himself march up each of these consecutively and defeat the two fractions of his enemy in detail. That is the simplest possible case, and it can be developed into any amount of detail and intricacy. The bridge is the point where the commerce up and down stream crosses the road-commerce transversely to the river-commerce, and the nodal point of the bridge establishes a market. But that nodal point has other characters even more important to civilian life. It creates a point of _trans-shipment_, where goods must be transferred from the water vehicle to the land vehicle. In their transference you have the political opportunity of examination and toll, and, if necessary, interception; and you also have, of course, the whole of the middleman business of dealing with and passing through the goods--you have the depot and the warehousing and all the adjuncts of a built-up commercial centre and a market.