However in July 1870 Napoléon III would enter into a war with Prussia with no allies and an inferior army.
Oops.
I find the Franco-Prussian War to be fascinating. When most present day people hear of a war between France and Germany they just immediately assume that Germany will of course win but at the time things were very different. The "German" part of the war wasn't modern unified Germany but rather the "North German Confederation which was less populous than France. Additionally, the Napoleonic Wars of the early nineteenth Century had only concluded about 55 years before. For a modern comparison, 55 years ago today was 1967 or the height of the Vietnam War. Thus there were still a LOT of veterans of the Napoleonic wars in the various countries of Europe and those wars were still very much in the public consciousness.
I can't find a citation for it offhand but I've read that London's Newspapers were printing maps of "Probable French Invasion routes to Berlin" at the beginning of the war. this is very interesting because Britain was neutral so these were not biased in favor of France. If anything the British were more likely to be biased AGAINST France due to the history (Napoleonic Wars and prior) and the fact that France and decidedly NOT Germany was seen as Britain's chief rival.
The results of the Franco-Prussian War were far-reaching and, to an extent, can still be felt today. France was not just defeated but outright humiliated by the Germans and the victorious Bismark was able to leverage that into unification of nearly all German people under one nation for the first time in history. The notable exception were those Germans living in Austria but the Austrian Germans ended up fighting on the same side as the rest of the Germans in WWI and birthing the man who would become fuhrer of a united (post Anschluss) Germany in WWII.
Britain's long-time (literally centuries) rivalry with France cooled in the face of Germany as the new dominant power in Europe which led to Britain moving closer diplomatically to France and ultimately lead to Britain and France standing together in both World Wars of the Twentieth Century.
France realized the need for allies in any potential conflict with the neighbor that was now stronger than they were which led to France dropping their conflicts with Russia (mostly dealing with Black Sea access) and cultivating improved relations with Germany's Eastern neighbor for mutual defense against potential German aggressions.
Lessons from the war (some very wrong) were also important to later events. Specifically, the end of the American Civil War had seen significant use of trench defensive fortifications and the horrors of static trench warfare began to be realized. However the Franco-Prussian War was extremely short. The whole thing only lasted barely over six months and for all practical purposes it was over with the surrender of Metz barely three months after the outbreak of hostilities. Thus the power of the offensive was idolized and the lessons of the American Civil War were lost. This led to literally thousands of unnecessary deaths in WWI as officers schooled in this belief in the offensive sent soldiers on hopeless headlong charges against entrenched defenders.
