As the Third Reich rolled across Europe, Lahner joined many artists who took refuge in Vichy France in the Dordogne. He stayed with the Averseng family in their chateau and traveled throughout the region. He seemed particularly impressed with the town of Collogne-la-Rouge, where he painted a large number of luminous canvases of its remarkable sandstone buildings.

Having been educated as a mining engineer he also explored Dordogne's famous caves and studied the primitive art covering the underground walls. In later years he incorporated these primitive elements into a series of paintings.

After the liberation of Paris in 1945 Lahner returned to rue des Perichaux where he began to exhibit his work more frequently. The post-War era was not only one of steady work for the artist, but also one of extensive travel. In 1948 he made his first of many trips to Algeria.