I really enjoyed reading Charles Glass’ book “American in Paris, Life and death under Nazi Occupation 1940-1944”(published by Harper Press). 
This is a story about a small group of unusual people, who found themselves in profoundly unusual circumstances. Yet it offers real insights into ordinary human nature, into how some people will optimistically collaborate with power, while others will resolutely resist, and how it is not always easy to predict in advance who will take which course. 
When France was first over run by the Germans in June 1940, the United States was still a neutral country and many Americans, who had put down roots in France ,felt under no particular pressure to leave. Some were married into French families and other had business interests in Paris. In fact, American citizens initially enjoyed a relatively privileged existence. The German authorities wanted the US to stay out of the war, and treating Americans in France well made sense for them.

In contrast, British, Canadians and other whose countries were at war with Germany since September 1939 were under severe restrictions and many were interned in camps at Compiegne and Vittel.

That changed when America entered the war against Germany in December 1941, but, even then, the French Government at Vichy, which had signed an armistice with the Germans in June 1940 and  administered some of the country, was not at war with the US, and the US continued to have an Embassy at Vichy 
Some American citizens had fairly close links with the Government at Vichy. One American citizen, Rene de Chambrun, was married to the daughter of Pierre Laval, the Vichy Prime Minister.

Another, Charles Bedaux, provided premises for the American Embassy after the French Government had fled Paris, but also enjoyed close business and social links with figures in the Vichy regime and had business contacts with the Germans. Others, like Dr Sumner Jackson of the American hospital in Paris, took the Allied side even before the US entered the war, and helped the French Resistance in many ways under occupation. Yet others, like the bookseller Sylvia Beach, first publisher of some of James Joyce’s work, kept their heads down and tried to survive.
This book brings out the many difficulties of ordinary life under occupation….the endless compromises, the lies, the black market, rationing, and the scramble for food.

City dwellers in France, in 1943/4, were almost starving.  The food needs of German forces got priority.

My own aunt, Hilda Delany, was a young nun in occupied France during the war, and it is widely believed in our family that she never recovered from her privations, and that this accounted for her death at an early age in the 1950’s.