What type of population changes happened during World war 1?

During World War I, significant population changes occurred due to mobilization, displacement, and casualties. Millions of men enlisted or were conscripted into armed forces, leading to a demographic shift as many left their civilian lives. Additionally, the war caused large-scale displacement of civilians, particularly in Europe, as people fled combat zones. The conflict also resulted in staggering loss of life, with millions of soldiers and civilians killed, permanently altering the population structures in affected countries.