Now look back at the first diagram; there you have the line A-B, and there are nine units just able to hold it.
Suppose by killed and prisoners and wounded and disease the nine dwindle to six, then the line A-B can no longer be held. It means in practice that the six remaining would have to be grouped as in Diagram V or as in Diagram VI, and in any case there would be a bad gap, double or single, through which the enemy pressing from E would pierce. What can the general in command of the defence do when his force has thus dwindled?
Diagram VII. The defender has no choice but to fall back on shorter lines, such as F-G, which his remaining six units can just hold. If the six dwindle to four he must again fall back to a yet shorter line, C-D.
He has no choice but to fall back upon shorter lines. That is, having only six units left he must retire to some such point as the line F-G, Diagram VII, where his remaining six units will be just sufficient to hold the line, and if the six dwindle to four he must again fall back to a yet shorter line, such as C-D.
Note carefully that this does not concern proportionate numbers. We are not here considering the relative strength of the defence and of the offence; we are dealing with absolute numbers, with a minimum below which the defensive cannot hold a certain line at all, but must seek a shorter one.
Diagram VIII. The Germans are now holding, roughly, the line A-B, from the North Sea to the Swiss Mountains—500 miles long in all its twists and turns. If dwindling numbers force them to take up a shorter line they could either abandon Alsace-Lorraine and substitute C-G for C-B, or abandon most of Belgium and Northern France and substitute E-C for A-C. With still failing numbers they would have to take up the still shorter line F-B. It would be no shortening of the German line to fall back upon the Rhine, D-D-D.
Now that is precisely the state of affairs upon the French and Belgian frontiers at this moment. The Germans are holding a line, which is roughly that shown in Diagram VIII, between the Swiss mountains and the sea near Nieuport, the line A-B about 400 miles long in all its twists and turns. If their numbers fall below a certain level they cannot hold that line at all, and they must take up a shorter line. How could they do this? Either by abandoning Alsace-Lorraine and substituting C-G for the present C-B, or by abandoning most of Belgium and all northern France, and falling back upon the line Antwerp-Namur-The Ardennes and the Vosges, substituting E-C for A-C. With failing numbers they would have to take up a still shorter line from Liege southwards, just protecting German territory, the line F-B.
As for the line of the Rhine lying immediately behind F-B, the line D-D-D, it is a great deal longer than the shortest line they could take up. F-B, and though heavily fortified at five important points and with slighter fortifications elsewhere, it would need quite as many men to defend it as a corresponding line of trenches. Thus it would be no shortening of the German line to fall back upon the Rhine.