Immediately after the battle on the Lisaine the forces investing Belfort were increased to 27 battalions, 6 squadrons, 6 field batteries, 24 companies of fortress artillery, and 6 companies of fortress pioneers; 17,602 infantry, 4699 artillerymen, and 1166 pioneers, in all 23,467 men, with 707 horses and 34 field-guns.

The place was invested on the north and west by only a few battalions, and the main force was assembled to the south and east.

On January 20th the eastern batteries opened a heavy fire on PÉrouse. Colonel Denfert concluded that an attack was imminent, and placed four battalions of his most trusted troops in the village, which had been prepared for an obstinate defence.

At about midnight, two battalions of the 67th Regiment advanced from ChÊvremont on the Haut Taillis wood without firing a shot. Once inside it there was a determined struggle, but the French were driven back on the village, and the pioneers immediately intrenched the skirt of the wood towards PÉrouse under a heavy fire from the forts. Half an hour later two Landwehr battalions advanced from Bessoncourt to the copse on the north of the village. They were received with a heavy fire, but made their way onward over abatis, pits and wire-entanglements, driving the enemy back into the quarries. A stationary fight now ensued, but the 67th presently renewed the attack, and without allowing themselves to be checked by the earthworks forced their way into PÉrouse. They took possession of the eastern half of the straggling village at about half-past two, and the detachment defending the quarries, finding itself threatened, retreated. At five o'clock, Colonel Denfert abandoned the western part of the village, which was now completely occupied by the Germans. The losses on the German side were 8 officers and 178 men; the French left 5 officers and 93 men prisoners.

January 21st to 27th.—The next day the construction of the first parallel was undertaken, extending about 2000 yards from Donjoutin to Haut Taillis. Five battalions and two companies of Sappers were employed in this work, and were undisturbed by the French; but the rocky soil prevented its being constructed of the prescribed width.

General von Tresckow considered that he might thus early succeed in carrying both the Perches forts by a determined assault. Two half redoubts with ditches more than three yards deep cut perpendicularly in the solid rock, casemated traverses and bomb-proof blockhouses in the gorge, afforded protection to the defenders. Each work was armed with seven 12-cm. cannon, and they were connected by trenches, behind which reserves were in readiness. On the right flank this position was protected by a battalion and a sortie-battery in Le Fourneau; on the left the adjacent wood was cleared, cut down to a distance of 650 yards, and wire-entanglements between the stumps formed an almost impenetrable obstacle. In front the gentle slope of the ridge was under the cross-fire of the two forts.

When on the previous evening of the 26th the construction of the parallel was sufficiently advanced to allow of its being occupied by larger detachments, the assault was fixed for the 27th. Two columns, each of one battalion, one company of Sappers, and two guns, passed to the attack at daybreak on that morning. Two companies of SchneidemÜhl's Landwehr Battalion advanced against the front of Basses Perches and threw themselves on the ground within from 65 to 110 yards of the work. A sub-division of sharp-shooters and a few pioneers reached the ditch and unhesitatingly leaped in; the two other (Landwehr) companies, going round the fort by the left, got into its rear, and here too the men jumped into the ditch of the gorge. But the French who had been driven out of their shelter-trenches were now assembled, and the battalion from Le Fourneau came up. All the forts of the place opened fire on the bare and unprotected space in front of the parallel, and an attempt of reinforcements to cross it failed. The 7th Company of the Landwehr Battalion was surrounded by greatly superior numbers, and after a brave struggle was for the most part made captive. Most of the men in the ditch were still able to escape.

The advance of the right column against Hautes-Perches also failed. It had to cross 1100 yards of open ground. The encompassment of the fort was attempted, but it was impossible to force through the abatis and other obstacles under the destructive fire of the enemy.

This abortive attempt cost 10 officers and 427 men; and the slower process of an engineer attack had to be resumed.

January 28th to February 15th.—As the approaches to the forts progressed the flying sap could be carried forward about 330 yards every night unopposed by the enemy. In spite of all the difficulties caused by the nature of the soil, on February 1st the second parallel was thrown up at half distance from the Perches.

As the Fort of la Justice was a special hindrance to the operations, two new batteries had to be constructed to the east of PÉrouse against it. Four mortar-batteries on the flanks of the parallel now directed their fire on the Perches at very short range. Three batteries were also constructed in the Bois des Perches to fire on the citadel, and one on the skirt of the wood near Bavilliers against the defences of the city. Henceforward 1500 shells a day were fired on the fortress and its outworks.

But further the prosecution of the attack became more and more difficult. The withdrawal of General Debschitz had seriously reduced the working strength of the besieging force. There were only nine battalions for the exhausting service in the trenches. Specially serious was the heavy loss in pioneers, and two fresh companies had to be brought up from Strasburg. The bright moonlight illuminating the fields of snow far and wide made it impossible to proceed with the flying saps. Sap-rollers had to be used; the heads of the saps had to be protected by sandbags and the sides by gabions, while the earth for filling had often to be brought from a long distance in the rear.

On the head of all this, on February 3rd, a thaw set in, and the water from the heights filled the trenches, so that all communication had to be carried on across the open ground. Torrents of rain damaged the finished works; the parapet of the first parallel gave way altogether in places, and the banquette was washed away. The bottomless tracks made the arming of the batteries unspeakably difficult, and the teams of the columns and field artillery had to be employed in bringing up the ammunition. Many guns had become useless by overheating, while the enemy understood, by rapidly running out their guns, firing, and then running them back again, how to interrupt the work. Not merely was it necessary to continue the shelling of the Perches during the night, but a brisk rifle fire had to be kept up against them. Only now and then did the batteries newly placed in the parallels succeed in entirely silencing the guns of Hautes Perches. Epaulments had to be erected against Fort Bellevue and the defences of the railway-station, and Fort des Barres resumed activity. That under such exertions and the abominable weather the health of the troops suffered severely, need not be said; the battalions could often only muster 300 men for duty.

Meanwhile, however, the artillery of the attack had unquestionably become very much superior to that of the defence, and, in spite of every obstacle, the saps were pushed on to the edge of the ditch of Les Perches.

On February 8th, at one in the afternoon, Captain Roese had gabions flung into the ditch of Hautes Perches, sprang into it with five sappers, and rapidly scaled the parapet by the steps hewn in the scarp. He was immediately followed by the trench guard, but only a few of the French were surprised in the casemated traverses. The situation of the garrison of the forts had in fact become extremely difficult. Ammunition had to be brought up under the enemy's fire, water could only be had from the pond at Vernier, and cooking could only be done inside the works. Colonel Denfert had already given orders to bury the material. Unseen by the besiegers the guns of which the carriages could still be moved had been withdrawn, and only one company left in each fort, which in case of a surprise was to fire and fly. Nothing was to be found in the abandoned work but wrecked gun-carriages and four damaged guns. This fort was at once reversed so that its front faced the fortress, but at three o'clock the latter opened so heavy a fire on the lost positions that the working parties had to take shelter in the ditches.

The garrison in Basses Perches attempted some resistance, but under cover of a reserve it soon retired to Le Fourneau, leaving five guns and much shattered material. Here also the fire of the place at first compelled the working parties to break off, but four 15-cm. mortars were at length brought into the fort, and two 9-cm. guns were placed on the spur of the hill to the westward, which directed their fire on Le Fourneau and Bellevue. During the night of the 9th the two works were connected by a shelter-trench 680 yards long, and thus the third parallel was established.

The position was now such that the attack could immediately be directed on the citadel, and on it the batteries in the Bois des Perches and presently those in the second parallel opened fire. Moitte, Justice, and Bellevue were shelled simultaneously. General von Debschitz had returned, so that the investing corps was thus again brought up to its full strength, and all the conditions were improved by the return of the frost. By the 13th ninety-seven guns were ready in the third parallel.

The town had suffered terribly from the prolonged bombardment. Nearly all the buildings were damaged, fifteen completely burnt down, and in the adjoining villages 164 houses had been destroyed by the defenders themselves. The fortifications showed not less visible indications of serious damage, particularly the citadel. The hewn-stone facing of its front-wall had crumbled into the ditch. Half of the mantleted embrasures had been shattered, the expense powder magazines had been blown up, and a number of casemated traverses pierced. The guns in the upper batteries could only be reached by ladders. The garrison, of its original strength of 372 officers and 17,322 men, had lost 32 officers and 4713 men, besides 336 citizens. The place was no longer tenable; besides there now came the news that the army from which alone relief was to be expected, had laid down its arms.

Under these circumstances General von Tresckow summoned the commandant after a defence so brave to surrender the fortress, with free withdrawal for the garrison, this concession having the sanction of his Majesty. The French Government itself authorized the commandant to accept these terms. Colonel Denfert, however, insisted that he must be given a more direct order. To procure this an officer was sent to Basle, pending whose return there was a provisional armistice.

On the 15th a convention was signed at Versailles, which extended the armistice to the three departments which till then had been excluded from it, and also to Belfort; but the 1st article demanded the surrender of that place.

After the conclusion of the definitive treaty, the garrison, in the course of the 17th and 18th, with its arms and trains, left the precincts of the fortress and withdrew by way of L'Isle sur Doubs and St. Hippolyte into the country occupied by French troops. The march was effected in detachments of 1000 men at intervals of 5 km., the last of which Colonel Denfert accompanied. The supplies which remained in the fortress were conveyed in rear of the departing troops in 150 Prussian proviant waggons. At 3 o'clock in the afternoon of February 18th Lieutenant-General von Tresckow entered the place at the head of detachments from all the troops of the investing corps.

There were found 341 guns, of which 56 were useless, 356 gun-carriages, of which 119 were shot to pieces, and 22,000 stand of arms, besides considerable supplies of ammunition and provisions.

The siege had cost the Germans 88 officers and 2049 men, 245 of whom were released from imprisonment by the capitulation. Immediately was set about the work of restoring and arming the fortress, and of the levelling of the siege works.