After a series of confusing diplomatic discussions between France and Prussia over the succession to the Spanish throne. The French became convinced the Prussians at an already nervous time had insulted them and so they declare war under the assumption they would win an easy victory.
Invasion of Belgium
French plans drafted during the Luxembourg annexation crisis called for a quick hook through Belgium bypassing the Rhine and the other defences as well as the bulk of the Prussian army.
The Prussians were aware of the French plan of attack and had prepared their own offensive accordingly. The Prussians would mobilize quickly and once the French attack started one force would allow them to cross Belgium and then defeat them when they entered Germany and they would then attack France.
After the declaration of war the Prussians mobilized rapidly while the French were slow in calling up their reserves and by the time they were ready 2 of the biggest battles were done.
100,000 French troops on the 29th June crossed the border from France to Belgium and started their march. The Belgian army was small and was having similar call up issues to the French and complained sternly about the breach of their neutrality. Yet they decided they should not try and acitvely block the French. On the 31st July the French crossed from Belgium into Northern Germany.
Within hours of them crossing they were faced by 200,000 Prussian troops and within hours had them encircled as they were caught in a pincer grip due to lack of reconnaissance. The French found themselves trapped in Stadtkyll and the surrounding areas. Napoleon ordered his men to break out but failed to escape the tight encirclement. On the 3rd August having failed to break out and with limited supplies the French army under Napoleon surrendered.
When Lepold II heard news of the battle and surrender he declared war on France and moved his troops to the border.
March on Metz
While back in Paris with news of their defeat their was an outrage at the defeat and the monarchy was thrown out and Third French Republic was established and they decided to continue the war with France. They ordered the remaining troops to stay on the defensive and keep them away from Paris.
The Prussians and Germans moved back to the French- German frontier and on the 8 August the Prussians launched their offensive across the French frontier and the French fall back as the Germans attempt to encircle and destroy them. They started retreating back to Metz and the Prussians aggressively pursued. After a 2 day pursuit the Prussians arrived at Metz to find the fort empty and the French had again refused to give combat.