Why was Germany unified in Versailles not Berlin?

Question

Why did Wilhelm I (or Bismarck) decide to perform official Germany unification in France?

In my opinion, if the ceremonial was to honour the Prussian king and be a tribute or allegiance to him, it should be held in Berlin/Potsdam (like cardinals honour new pope in Rome).

Of course, capturing Paris in 1870 war was a spectacular event showing the Prussian dominance, however (as it seems) the French capital was not considered to be kept by the Prussians (or Germans) forever.

I performed some research, however I couldn't find any detailed information, nor any trustworthy sources.

Bismarck's final step to complete unification was to challenge the power of France on the southern border. Since Richilieu and Louis XIV, France had made a divided Germany a prime component in French foreign policy. Bimarck would have to tread carefully if he were to unify the scattered Germans. [source]

This above suggest it was to show France that "what Richelieu and Louis XIV did" was now aborted. The best place to show something to France would be Paris.

This paper in some way suggest the creation of the German Empire is related to Charlemagne:

The Franco-Prussian War of 1870-1871 was reflective of the conflict between the western part of the Charlemagne Split (France) and the eastern part of the Charlemagne Split (Germany). The powerful German state of Prussia unified all of the German states and built the Second Reich (...)

Maybe it was because everyone was in hurry, and Wilhelm was visiting the General Staff. The idea might be backed by the webpage of the Versailles palace:

On 16 December 1870, a delegation from the Parliament of Northern Germany arrived in Versailles. It came to beseech the king of Prussia to accept the title of Emperor of Germany. The Confederation was dissolved on the 20th. The proclamation of the Empire was fixed for 18 January 1871 in the Hall of Mirrors. An altar was set up here for the religious ceremony. A stage was installed along the side next to the Salon of War, facing the spot where the throne of Louis XIV stood. 600 officers and all the German princes were present except Louis II. After the Te Deum, Bismarck, in his cuirassier’s uniform, read out the proclamation. When he had finished, the Grand-Duke of Baden shouted “Long live his Majesty the Emperor William!” The room rocked with the assembly’s “hurrahs!”. The Chancellor had finally made his dream come true under the paintings of Le Brun glorifying the victories of Louis XIV on the Rhine. He had also achieved his revenge for the defeat of Iena in 1806. The Germans soon left Versailles to the elected representatives of defeated France.

Answer 1

Until 1871, Germany wasn't "Germany." It was a collection of (often) warring German-speaking states like Prussia, Bavaria, etc. Austria, which was occupied elsewhere, never did join.

The thing that unified the "Germans" was their common distrust of the French, even though some German states liked France more than others. Essentially, uniting to defeat and keep down the French was the raison d'etre for creating a united Germany. The coronation of Wilhelm in the Versailles was just a celebration and reminder of that fact.

On the other hand, having a coronation of Wilhelm in Berlin would have indiscreetly "highlighted" the absorption of smaller German states into a Prussian-led union. That was a symbolism that e.g. Bismarck was anxious to avoid, even though that was actually what was taking place.

Put another way, the "German" states united more because they were anti-French than they were pro-German.

Answer 2

The Constitution of the German Confederation (1871), that turned the North German Confederation and several South German states into the German Empire was enacted on January 1st, 1871. The proclamation of Wilhelm I. as Emperor on January 18th was a "taking of office", not the unification per se.

Declaring Wilhelm I. Emperor in the Hall of Mirrors served several purposes:

• The Hall of Mirrors is at the very heart of France. This symbolized the birth of the German Empire on military success over the "Erbfeind" (hereditary enemy). The hall is decorated with pictures celebrating successes of Louis XIV of France, including territorial gains over previously German territory (Alsace, Freiburg, Kehl and others), reinforcing the symbolism.

• To give the ceremony the expected gravitas, regiments from most constituting German states assembled in full parade uniform, displaying their banners. These troops were "at hand". Note that the Franco-Prussian War was not even over yet. Having a similar ceremony in Berlin (or elsewhere in Germany, not that Wilhelm I. would have liked that idea...) would have meant delaying the ceremony, and probably would have made the protocol of getting the troops assembled for such a display much more difficult. The "high point" in public perception would have passed.

• As @TomAu pointed out, one of the more delicate points was how to not offend the other constituting states by too open a display of Prussian predominance. Wilhelm I., for one, wanted the title "Emperor of Germany", whereas Bismarck wanted the much more appeasing "German Emperor". That particular dispute on what exactly the title was to be wasn't even settled at the point of proclamation -- Wilhelm I. was hailed as "Emperor Wilhelm", side-stepping the whole issue (which was later resolved on Bismarck's terms). You can imagine how much the South German states would have liked having to hail their new Emperor in Berlin, the Prussian capital...

Answer 3

I think you already answered your own question. It was more fortuitous than anything else: that's where everybody was when the time came. Remember that Germany had already been long united in the Holy Roman Empire, which the French has destroyed in 1806. In the Franco-Prussian war of 1870, the Prussians reversed that dominance, and created the grounds for the re-establishment of the Reich. King Wilhelm of Prussia led the Prussian army personally to victory. When the battle was won, all the various German princes flocked to him and the second reich was created. If he had been in Konigsberg instead, they would have gone there.