In 1870 he suffered a major reversal. Napoleon III had declared war on Prussia with disastrous results. Paris was under siege for 4 months and in December, starving citizens invaded Monier's property and removed everything edible, including the horses. His caretaker died attempting to resist them. In January 1871, the Prussian bombardment ruined what was left. Monier and his family clung on through the severe winter. Though peace was declared in March, the citizens of Paris refused to concede. Monier and his workers started rebuilding the business under the rigours of the Commune.