The German-French border underwent several revisions in the period between 1870 and the present day. Following the Franco-Prussian war from 1870-71, the French ceded large parts of the Alsace and Lorraine regions to the newly formed Germany. You can see the region highlighted in red [here](https://en.wikipedia.org/wiki/Alsace-Lorraine#/media/File:German_Empire_-Alsace_Lorraine(1871). Alsace-Lorraine, as it was then named, was administered as an 'Imperial Territory', under the administration of the Reich Government in Berlin, rather than being granted self-rule. The population resented this, and the Alsace-Lorraine party would regularly gain 10-15 seats in Reichstag elections. Tensions culminated in the Zabern Affair in 1913, where heavy-handed treatment of Alsations by German troops led to mass protests and escalated to a vote of no-confidence in the Chancellor.

After the end of the First World War, Alsace-Lorraine was returned to France by the Treaty of Versailles. Unlike in the East and North, there was no plebiscite to allow the Alsatians to decide their own future. In addition to this, the Saar region of Germany became a League of Nations mandate under the government of France and Germany. The region occupied was roughly the same shape as the Saar today. The Saar was only restored to Germany in 1935, after a plebiscite on rejoining the country was held and won by a considerable margin.

During World War 2, Alsace-Lorraine was informally annexed to Greater Germany following the occupation of France. However this was reversed following Germany's defeat. The Saar region was also detached as a French protectorate, and moves were made to annex it or keep it independent by the French, but it was re-absorbed into West Germany in 1957.

To sum up as a short timeline:

1871: Germany annexes Alsace-Lorraine.

1919: France regains Alsace-Lorraine, the Saar becomes a League of Nations Mandate

1935: Germany regains control of the Saar.

1940: Germany informally annexes Alsace-Lorraine, this doesn't matter as Germany also occupies France anyway.

1945: France regain Alsace-Lorraine, Saar held as a protectorate.

1957: West Germany regains control of the Saar.

I hope this helped, let me know if you have any more questions.