Here's the occurrence of the noun
devoir (meaning duty) in a corpus of French books from 1800-2008:
There are big spikes during major wars. The other place with a noticeable spike is around the 1848 revolution, interestingly.
For comparison I used the verb
devoir (to have to). It doesn't have such obvious spikes.
Theory suggests that war could cause to the evolution of human in-group altruism, and evidence shows that conflict indeed makes people more prosocial. This chart shows one channel for that to happen: changes in social norms.
The picture for Germany is not so clear. Germany has two words for duty,
Pflicht and
Aufgabe.
Aufgabe also means "function", so it's more ambiguous.
Here's the graph:
There are spikes in
Pflicht around 1871 and 1914-18, but not noticeably for 1939. And for
Aufgabe, the big jump comes not in 1939 but in 1933, when the Nazis took over. The early data for Germany also seems more random (though this could
relate to the Napoleonic wars, which were a crucible of German
nationhood - actually, that's true of France also, but the big early
spike in the graph above is around 1820 not 1815).
Lastly the UK - this is the "British English" corpus:
The spikes in the 19th century are around 1830 and 1853. 1830 is a time of civil unrest just before teh passing of the first Reform Bill. 1853 is the Crimean War. 1914-18 shows up as a spike but only at about 1916. And in 1939 there's a wee spike that quickly vanishes... it's easy to fit a
post hoc interpretation about the un-idealistic spirit with which Britain approached World War II.
OK, so using the word "duty" in war may not be such a big surprise. What about the most basic group-word, "we"?
Here's France:
Clear spikes during the three major wars.
Here's Germany. (Sorry about the unhelpful x axis labels, blame Google. The lines are 20 year intervals.)
There's a huge spike for WWII and a spike for 1914-18. Actually, the WWII spike is 1945 precisely. A null for 1871. (Earlier on there's something about the 1840s... here my knowledge of European history is lacking!)
Britain is less clear. There are spikes for the two world wars but they stand out less, especially compared to the sustained height of the 19th century:
All of this is very quick and dirty, but I think there is scope for more exploration.