This fortress, lying in the plain and of very symmetrical shape, was proof against a coup-de-main because of its ditches, which were dry indeed, but faced with solid masonry. The garrison of over 5000 men had well-protected quarters in the bomb-proof casemates of the ravelins. Fort Mortier, standing near the Rhine, and constructed for independent defence, effectually commanded the ground over which the intended attack must be made on the north-west front of the fortress. Therefore 12 heavy guns were brought up from Rastatt to Alt Breisach, where the right bank of the Rhine commands the fort at effective range.

It was not till near the end of October that the siege-guns arrived before New Breisach from Schlettstadt, and when the infantry had closed up and all preparations were complete, fire from 24 heavy guns was opened on the fortress on November 2nd from Wolfgantzen, Biesheim and Alt Breisach.

By three o'clock a large part of the town was on fire, and detachments of infantry were skirmishing with the French posts at the foot of the glacis. Fort Mortier had suffered exceptionally severely. Nevertheless, an attempt to storm it was repulsed, but at six o'clock it capitulated, an utter ruin. Only one gun remained in serviceable condition. Two new mortar batteries were erected to shell the main fortress, the defence became perceptibly more feeble, and on November 10th Breisach surrendered on the same terms as Schlettstadt, but the garrison was allowed to march out with the honours of war. The fortifications were almost uninjured, but the town was for the most part burnt down or severely damaged. The success had cost the Germans only 70 men; 108 guns, 6000 small arms and large quantities of stores fell into their hands.

While these strongholds in Alsace-Lorraine were thus being reduced, Verdun still intercepted the line of railway which formed the shortest line of communication with Germany.