Historically speaking, there are in Alsace-Lorraine three parts: there is Lorraine, there is Alsace, and there is the southern part of Alsace including the town of Mulhouse.
As regards the town of Mulhouse, the question is most simple and clear. The town never, at any time, belonged to Germany or to the Germans. It belonged to Switzerland and, at the end of the 18th century, during the French revolution, the town, after a referendum, decided to become French. A delegation was sent to Paris, to the French Parliament, then called the Conseil des Cinq-Cents, and the delegation expressed publicly, officially, the desire of Mulhouse to be part of the French territory. There was a deliberation, and unanimously the Conseil des Cinq-Cents voted a motion couched in the following terms: "The French Republic accepts the vow of the citizens of Mulhouse."
A few weeks later the French authorities, among scenes of unparalleled enthusiasm, made their entry into the town, and the flag of Mulhouse was wrapped up in a tricolor box bearing the inscription: "The Republic of Mulhouse rests in the bosom of the French Republic."
Alsace—the rest of Alsace—became French in 1648, more than two centuries before the war of 1870. It became French according to a treaty. The treaty was signed by the Austrian Emperor, because Alsace belonged to the Austrian Imperial Family. And it is not without interest to quote an article (article 75) of the treaty:
The Emperor cedes to the King of France forever, in perpetuum, without any reserve, with full jurisdiction and sovereignty, all the Alsatian territory. The Austrian Emperor gives it to the King of France in such a way that no other Emperor, in the future, will ever have any power in any time to affirm any right on these territories.
When today one reads that treaty, one has the impression that more than two centuries ago the Austrian Emperor had already a sort of apprehension that later on another Emperor would interfere in the matter and create mischief!
Fifty-three years after that treaty, the Prussians, who dislike seeing anything in some one's else possession, tried to recover Alsace. Their own ambassador tried to dissuade them, and in 1701 Count Schmettau, ambassador of Prussia in Paris, wrote to his king:
"We cannot take Alsace, because it is well known that her inhabitants are more French than the Parisians...."
Could anything answer better the affirmation that "Alsatians are of German tendency?"
Lorraine became French in 1552, more than three centuries before the war of 1870. Lorraine became French not after a war and as the result of a conquest, but according to a treaty signed by all the Protestant Princes of Germany, in which we find the following sentence, which is really worthy of meditation: "We find just that the King of France, as promptly as possible, takes possession of the towns of Toul, Metz, and Verdun, where the German language has never been used." So that the Germans themselves put on the same line the towns of Metz, Toul, and Verdun, and recognized that the town of Metz was not German.