Jean-Pierre had worked as a locksmith in Frisange for two years when he was conscripted into the Army in August 1947.

At first, he joined the Corps de la Garde. He reported to the first contingent in late September 1950. In Korea, Jempi was wounded by a bullet or shrapnel from a grenade. After the projectile was removed from his body, he had it melt and shaped into a Buddha figurine, which he brought home from Korea as a souvenir. Following his term overseas, he was promoted to the rank of Caporal in December 1951. He stayed under arms until his military discharge in August 1953.