So far it would seem the US Civil war started on hard money and turned to fiat.
> Demands for money in the Civil War rapidly exceeded the availability of specie (gold and silver) and the capacities of the private banking system. The governments in both the North and the South began to print their own currencies as a temporary wartime expedient. In the Union, competing currencies were eventually curtailed by a 10% tax on privately printed notes. Many of the former note-issuing businesses successfully liquidated or amalgamated and others were converted into “national banks” under the National Banking Act of 1863.
Clearly that's just one case, but it will take me a while to gather further evidence.
Edit:
Seems federal reserve bills were in use through both World Wars while silver certificates dwindled out of existence as they were exchanged for silver.
> Following the financial panic of 1907, the Federal Reserve banking system was established to provide a safety “buffer” between the government and the economy. A new Federal Reserve Bank Note currency was able to be sustained through the Great Depression of the 1930s while most other forms of American currency were abandoned
So from a United States Centric view, fiat took us through some large wars.
I'll check on some other countries, but at this point I think it's fair to ask you to provide some evidence to back your statement.
> In fact, all of the biggest wars were without fiat money
I was more thinking of pre-20 century wars. However, it is true that world wars caused countries to shift away from gold. After wwi many countries did return (british pound, german mark), but after wwii they had a more permanent switch.
I will try to do some reading.