They were decisive and important because they achieved strategical purposes vital to the Allied campaign. Let us try to make that clear.
When after the defeat on the Marne the Germans took up their new line from the north of the Aisne to the Argonne, their utmost energy and resource were put forth to send into the fighting line from Germany fresh reserve formations which would give their forces not only a numerical but a military superiority.
But the effect and value of those fresh masses clearly depended on their being employed at the decisive points. Where were those decisive points?
The decisive points were first the extreme left of the Allied line, where it turned round from the north of the Aisne to the Oise, and secondly Verdun and along the eastern frontier.
Consider the effect had the Germans been able promptly to throw decisively superior forces against the Allies at those points. They would have turned both flanks of the Allied line, they would have forced a general retreat, and they would have been able once more to resume the offensive, but this time probably with the fortified frontier in their hands.
There can be no doubt that, broadly, that was their intention; and it was plainly seen by General Joffre to be their intention, because eastward from Rheims to the Argonne in their fortified line across the highlands the Germans remained from first to last upon the defensive.
This, however, was the situation the Germans had to meet: between the Aisne and the Oise a new and powerful French army under the command of General de Castlenau; on the Aisne and round Rheims, a tremendous and sustained onset by the 6th French, the British, and the 5th French army; between Rheims and the Argonne, an offensive which pushed them successively out of Suippes, and Souain, and therefore off the great cross-roads; in the Argonne, an offensive which forced them back from St. Menehould and beyond Varennes, and closed the defiles; round Verdun, and in the Woeuvre, an onset which threatened to cut communications with Metz.
Now the effect of these operations was, among other things, to restrict the German means of movement and supply; and it was a consequence of that restriction that even though there might be two or more millions of men then ready in Germany to be sent forward, there were neither roads nor railways enough to send them forward save after delay, nor roads or railways enough to keep them supplied when they had been sent.
With the means at their disposal—those means were still great, though not great enough—the German Government had to choose between various alternatives. As to the choice they made, later events leave no doubt. They sent forward troops enough to defend their flank between the Aisne and the Oise—it was all at the moment they could do; and they employed the best and heaviest of their masses of reserves partly to resist the British attack, but mainly to resist the 5th French army. At this time they had to let the position in the Argonne, round Verdun, and on the eastern frontier go; that is to say, they had there to remain for the time being on the defensive.