During the invasion and occupation of 1914-1918, German soldiers had sexual relations with local women in Belgium and France. There were many cases of rape during the invasion, but the occupation itself was more characterized by a rise in prostitution. Other forms of intimate relations also emerged, but they generally did not outlast the war itself.
Debruyne, E. (2016). Intimate Relations between Occupiers and Occupied (Belgium and France). In Ute Daniel, Peter Gatrell, Oliver Janz, Heather Jones, Jennifer Keene, Alan Kramer, et Bill Nasson (ed.), 1914-1918-online. International Encyclopedia of the First World War. Freie Universität Berlin. https://doi.org/10.15463/ie1418.10919