All I know as yet about Weiss’s war experiences comes from a brief entry (in poor English) from Wikipedia, which rather than summarise I shall just quote here:
In 1913 he finished the Gymnasium in Hradec Králov and since 1913 studied law in Vienna, but in 1914 he was forced to go and fight in World War I, after studiing two semestres. Here, in 1916 by he was captured at Tarnopol and spent his time in two prisoner camps in the Siberia, where he contracted Typhoid fever. (Note: there exists a rumor that he was captured on the Italian front, but this is not true). In 1917 he was transferred to the camp in Berezkova, where they amputated his frozen thumbs. Here he joined the Czech Legions. It is known that the dealings of detainees who caught this disease were the main inspiration for his earlier works, most notably Barák Smrti. After 1919 he joined the Czechoslovak legions. He returned to Czechoslovakia in February 1920 (actually visiting this country for the first time, as he was born in Austria-Hungary, in the Bohemian Kingdom.
I hope to be able to improve on this in due course.