Who was German's leader during WW1?
Kaiser Wilhelm the Second. Kaiser (derived from Caesar) was the title of the German and Austrian emperors.
The assumption made in the question is not accurate. The Germans annexed Alsace and Lorraine from France in 1871 after the Franco-Prussian War of 1870-71, mainly for strategic reasons and the area was heavily German-speaking. They did not take it during WW1, however. France annexed the territory after WW1.
Not at all. The "Nazi" political party (Deutsche Arbeiterpartei, later Nationalsozialistische Deutsche Arbeiterpartei) was created in 1919, after WW1, and nazism as an ideology didn't become important until the 1930s. Therefore, there is no reason at all why German soldiers should be called Nazis during WW1 (the French called them Bosche for some reason though). However, during WW2, many soldiers referred to Germans as "Nazis" in a derogatorily way even though not all Germans were…