Siege of Paris (1870–71) - Wikipedia

On September 19, 1870, the Siege of Paris by Prussian Forces begins and lasts until January 28 1871. From the article:

"As early as August 1870, the Prussian 3rd Army led by Crown Prince Frederick of Prussia (the future Emperor Frederick III), had been marching towards Paris. The army was recalled to deal with French forces accompanied by Napoleon III. These forces were crushed at the Battle of Sedan, and the road to Paris was left open. Personally leading the Prussian forces, King William I of Prussia, along with his chief of staff Helmuth von Moltke, took the 3rd Army and the new Prussian Army of the Meuse under Crown Prince Albert of Saxony, and marched on Paris virtually unopposed. In Paris, the Governor and commander-in-chief of the city's defenses, General Louis Jules Trochu, assembled a force of 60,000 regular soldiers who had managed to escape from Sedan under Joseph Vinoy or who were gathered from depot troops. Together with 90,000 Mobiles (Territorials), a brigade of 13,000 naval seamen and 350,000 National Guards, the potential defenders of Paris totalled around 513,000 personnel.[1] The compulsorily enrolled National Guards were, however, untrained."