It depends on what you mean when you speak about "Germans".

In history, "German" was the umbrella term for the languages and dialects spoken in large parts of the Holy Roman Empire (HRE), until it ceased to exist in 1806, when German states were occupied by Napoleon's troops. This changed when the people living in German states began to reinterpret (misinterpret would be more accurate) the history of the past ~1800 years for their own cause, in particular the history of the HRE, despite the fact that it was a multiethnic, supranational entity. Since then, German nationalists wanted German states to "reunify" to a new Empire, and their dreams did become true in 1871. Since then, people living in "Germany" were considered "Germans", but they kept the citizenship of the German state they lived in, e.g. Prussian, Bavarian etc. There was no "German" citizenship, until the Nazis defined it in 1934.

There are two regions that are part of contemporary France, but belonged to the HRE until the 17th/18th century AD, Alsace and Lorraine. After France lost the Franco-Prussian War (1870/71), Germany annexed these regions, which was occupied until it became a German state within the Empire in 1911. Since people in Germany were citizens of the state they lived in, the people living in Alsace-Lorraine were citizens of the the Alsace-Lorraine German state, or Reichsland Elsaß-Lothringen. In WWI, many Alsace-Lorrainians served in the German military, they were conscripted to serve in the Imperial Navy to avoid moral conflicts with French locals - their region was one of the main battlefields of WW1. After the abdication of the Emperor in 1918, the new state became independent, but was occupied by French troops within two weeks.

France established an assimilation policy. It began to categorize the approx. 1,8 million inhabtitants into four categories: A. French citizens (=everyone whose parents were born in either France or Alsace and Lorraine), B. partly French (one parent French or born in Alsace and Lorraine), C. Foreigners (from every state except Germany), D. Germans (everyone who moved from Germany to Alsace and Lorraine after 1871, these were approx. 100,000 people). The "Germans" (=members of the latter category) were forced to move back to Germany, but because of Woodrow Wilson's intervention, some were allowed to come back.

The French military retained the conscription in the interwar period, so men from Alsace-Lorraine were conscripted to serve in the French military, since they were citizens of France.

The Allied Forces occupied the Saarland and other parts of Germany west of the river Rhine after WW1, to force Germany to fulfill the Treaty of Versailles, and to avoid Germany's post-WW1 military buildup. The Saarland was among those regions Germany lost to France after WWI, and the French administration enforced the same assimilation policy as in Alsace-Lorraine. But since the region was part of other German states (Prussia and Bavaria held most parts of it) since the 18th century, the people living there had a predominantly "German" identity, and resisted the assimilation. They may also have resisted the draft by fleeing to Germany, which had no conscription 1918-1935.

Between 1919-1933, approx. 30,000 "Germans" (here, I mean people which were citizens of the German states, since there was no German citizenship yet) served in the Légion étrangère. In 1927, every second member of la Légion was "German". Most were WW1 veterans who had no place in the German military, which was reduced to 115,000 men by 1921 to fulfill the Treaty of Versailles. After 1933, many légionnaires fled from the Nazi dictatorship in Germany and served for France, in the colonies mostly. There were also many thousand Germans in the post-WWII Légion étrangère.