Well, Bismark didn't want to take ANY land from France in fear of French revanchism. However, nationalistic sentiment made him concede to annex the German-speaking part of Alsace, and the Moselle area of Lorraine was a great strategic position, and also had a German plurality, so it was annexed as well, forming the Reichsland of Elsaß-Lotharingen. No idea how to stop nationalism here without going one step further and annexing the entirety of both Alsace and Lorraine, which is something that would never happen. So, the peace was going to take Alsace-Lorraine, only way to prevent that is if Prussia and the German states lose the war, and considering they were allied with Russia at this time, I doubt it would happen. Besides, to beat Prussia, they would have to march all they way to Berlin, which would be extremely slow, and by the time they are in the vicinity the Italians would have joined and started attacking in Savoy and Provence. So, Alsace Lorraine is going to Germany unless France SOMEHOW manages to win that war, which, as I said, is unlikely since they are at a MASSIVE disadvantage.