Dachspmg, a good summary of Boney III. However there are several flaws he did nevertheless.
Indeed he felt he had to defend the Papal State because of the Catholic church in his country. That's why he lost Italy. His diplomacy of the 1860s was a series of defeats though.
At first his defeat in Mexico, where the French finally had to retreat. In Italy won the war of 1859, but on the second view not that impressing. Nevertheless the French soldier was recognized as the best in the world and the population was full of nationalistic dreams. So a strong German state was a nightmare to them.
Bismarck had managed to make treaties with Britain and Russia. Nappy III. couldn't prevent that. In the German War of 1866 he believed in a new 7 years war and even offered Bismarck help, if he was able to get the Rhine as border. He assumed he needed his help. But Königgrätz was the turning point. Austria was defeated in 7 weeks! But with a chancellor von Beust Austria wanted revenge.
In the mean time he tried to gain the Rhine as border and in the first step tried to annex Luxemburg and Belgium. That lead to yet another defeat. Although he thought he did with Bismarck's authorisation, he was wrong when Bismarck strongly opposed that. He outmanoeuvred Nappy III! So the French emperor had to suffer this defeat.
In the next time the North German Confederation was founded. This state did not include the South German states. However in secret treaties with them Bismarck managed to form an alliance against any attacker of Germany. The hatred against the French was higher than the hatred against the Prussians. Only Ludwig II. of Bavaria was a keen supporter of France. However, Bavaria also signed the treaty with North Germany. That he didn't see.
Bismarck however, knew he needed a war against France to fulfill the unification of Germany. That he prepared. And the next chance was an outstanding one: The Spanish throne was empty and a distant relative of Wilhelm I. was asked to become king. However this time Napoleon achieved a diplomatic victory- at first, when Wilhelm aborted this plan.
In 1870 Napoleon had many crises in France. He needed soon a thing to push up his popularity. And one good thing is bashing the Germans. So the French ambassador went to Bad Ems, where Wilhelm made holidays. So out of a victory you can make a stunning defeat! His behaviour lead to the so called Emser Depesche. This made public a day later, although slightly shortened to make it sound harsher than it was, provoked a French declaration of war against North Germany. Napoleon III. had hoped to silence this affront, but now he could not do this any more. He had to declare war on North Germany. Otherwise he would have lost his throne. Nevertheless he thought still to have the best army of the world. But indeed that was a mistake.
However even before the French invaded Germany, and they only planned for a war in Germany, even the families of the soldiers were allowed to accompany the soldiers (!), the diplomacy of Nappy III. got one defeat after another. Denmark got twice a bloody nose and only a fleet with an expeditionary corps would have changed that. In contrast of 1864 the North German fleet was able to break the Danish at once. And so the needed more than the fleet the French send. Although the German fleet was much smaller the fighting power of the world's biggest warship, SMS König Wilhelm, was frightening the whole French fleet. And as the expeditionary corps was needed in France, Denmark stayed neutral. Britain and Russia were also neutral and not willing to help the French. And Austria was keen to declare war on North Germany, but again wanted to prevent a 2 fronts war with Italy and insisted on Italian DoW on Germany! The Italians agreed, but only for the price of the Papal State! That, however, was not possible, as Napoleon III. feared for his throne if he did so. And the catholics were the last bastion of his power in France. At last the South German states declared war on France honouring the treaties with North Germany.
A diplomatic utter defeat for France. And the war itself lead to the end of Napoleon's rule. After the first invasion of German soil was beat the German forces invaded in full strength the Alsace. Weißenburg and Wörth were defeats for the French army of the Rhine. The whole French army was still in mobilising when these battles happened. Soon the Alsace was lost, and the incompetent Marshal Bazaine, a darling of the left as he started as private, was unable to retreat to MacMahon and the main forces of the French trying to build up a line to hold up the German juggernaut. But Bazaine only got to Vionville, a few km near to Metz, his HQ. There a single German corps was able to hold the lines and finally even to let the whole French army retreat. A few days later the enclosed army was beaten and had to surrender in the battle of Metz. Without these forces the French army marched east to the Belgish border, where at Sedan they were finally cought and after a battle had to surrender, too.
Napoleon became a PoW. And the next day he lost his throne. Although the French decided to fight on, soon Paris was enclosed and sieged, surrendering finally in January 1871 and thus ending the war.
Adler