Czechfag here. Frantisek Kupka, an early abstract painter, worked in France in 1914 and immediately volunteered for "Compagnie Nazdar", a Czechoslovak unit in French Foreign Legion. He served in combat until spring 1915 when he was sent back on medical grounds. Cubist sculptor Otto Gutfreund did the same and even survived company's annihilation at Arras in May of 1915, only to be banned from service a month later due to Béranger's Law (an order forbidding FFL from admitting citizens of enemy countries) and interned in a French POW camp until 1918.

Felix Holzmann, popular comedian from the 60s and 70s, known for his stage persona of a curious, annoying dimwit in a bucket hat, served in the Wehrmach during the war. Being ethnic German, he became a Reich citizen after German takeover of Czechoslovakia and got drafted into Coastal Artillery, where he served in Northern France and later in the Baltics. Spent at least a year in a Soviet camp after the war, never publicly spoke about any of that.