The siege of Danzig (18 March-27 May 1807) was the main French activity in the spring of 1807 and saw them capture one of the last strongholds in Prussian hands after the disastrous defeats of Jena and Auerstadt (War of the Fourth Coalition)
Although the main Prussian armies had been destroyed early in the campaign, Prussian resistance didn’t end. The King and Queen retreated in East Prussia, where they joined up with their Russian allies. A number of fortresses stayed in Prussian hands, including the important Baltic port of Danzig. Napoleon decided to conduct a winter campaign against the Russians, but this ended with the costly drawn battle of Eylau (8 February 1807). This was Napoleon's first real military setback on land, and left him in a potentially dangerous position isolated in eastern Europe. For the moment Austria was quiet, after her crushing defeat at Austerlitz, and the German states either defeated or allied with him, but further defeats might have inspired opposition. Napoleon thus spent much of the winter organising a new army in Germany, and finding replacements for his own army.
As part of the effort a new multi-national X Corps was formed, under the command of the veteran and respected Republican Marshal Lefebvre. The new corps had two Polish divisions, two Italian divisions, one French division (Menard), and troops from Saxony and Posen. As a new and untried unit it wasn't really suited for front line service, but Napoleon decided that it could handle the siege of Danzig. Chandler also suggests that Lefebvre was chosen to command the siege to give him a military victory that would justify his inclusion in Napoleon's new Imperial peerage, and he did indeed become Duke of Danzig after the siege. Lefebvre had 20,000 men from his own corps to conduct the siege, with Lannes and Mortier close enough to act as a reserve.
Danzig sat on the south bank of the River Vistula, close to the Baltic coast. The Vistula flowed west, parallel to the coast, then turned north at Danzig, split to flow around Holm Island and then into the sea at Neufahrwasser. The Royal Navy had a squadron in the Baltic, but Danzig was three miles inland, so command of the sea wasn't quite as vital as it could have been. Even so the French forces were split in two by the Vistula.
Danzig was defended by 14,400 infantry and 1,600 cavalry under the command of General Kalkreuth. He had 303 guns, 20 howitzers and 26 mortars under his command, and as a major port Danzig was filled with supplies.
Lefebvre was ordered to prepare for the siege of Danzig on 18 February. His troops advanced towards Danzig in early March, and drove back the Prussian outposts on 11 March. Danzig was formally invested on 18 March.
Lefebvre decided to make his main efforts to the west of Danzig. The first parallel was completed on 2 April and work began on the second parallel on 11 April. A second set of works were also built to the south-west of the city, The Prussians carried out a sortie on 11 April, but they were unable to slow down the work, and the second parallel was complete by 14 April. On 15-17 April fortifications were built north of the Vistula, facing the Prussian positions at the mouth of the river. On 24 April the heavy batteries were ready to open fire. A sortie on 26 April was unable to stop the French from completing the third parallel on 29 April. Finally, on 7 May, the French captured Helm Island, blocking the river route between Danzig and the coast.
This was just in time. On 10 May the Russians landed 8,000 men, commanded by General Kamenskoi, at Neufahrwasser on the coast. If the island had still been in Prussian hands then his task would have been easy, but its loss meant that the river route was blocked. Kamenskoi paused for four days, and this gave Lannes time to rush his nearest troops to the front. His first troops were in place by 12 May, and by the time the Russians finally attacked their route was blocked by forces under Generals Schramm and Gardenne.
Kamenskoi attacked on 14 May. He advanced along the narrow spit of land between the Vistula and the sea, hoping to recapture Helm Island. Lannes's leading troops managed to hold off the Russians long enough for Lannes and Oudinot to arrive with more troops. The attack failed and the Russians retreated having lost 1,500 men.
The garrison of Danzig had been curiously inactive during the Russian attack. They did carry out another sortie on 20 May but this was too late. On 21 May Marshal Mortier's corps joined the attacking forces, giving Lefebvre 47,900 men.
It was now clear that the fall of Danzig was only a matter of time. On 22 May Lefebvre sent an envoy with surrender terms, and Kalkreuth entered into talks. Napoleon was already planning to resume the offensive, with the start of his attack timetabled for 10 June, and so he was willing to offer generous terms. On 27 May the garrison of Danzig marched out with full honours of war, and they were then escorted to the Prussian outposts at Pillau, having agreed not to fight against the French or their allies for twelve months. Napoleon was now free to turn his attention to the Russians, but it would be the Russian commander Bennigsen who moved first, at the start of the campaign that led to the battle of Friedland.