In 1941 Diez de Medina submitted himself to trial after he was accused of selling life-saving Bolivian visas to up to 15,000 Jews in Europe during World War II.
He signed on 9 July 1925 the Carillo-Diez de Medina treaty with Argentine representative Horacio Carillo, which settled a long border dispute between Argentina and Bolivia.
The plan, which was to result from an American mediation between Peru and Chile, failed due to a change in U.S. foreign policy following the election of President Herbert Hoover.
Diez de Medina also enacted, together with Peruvian emissary M. Elias Bonemaison, the Treaty of 23 September 1902, which demarcated the border between Bolivia and Peru.
In 1941 Diez de Medina fell out of favor amidst a jingoistic political climate when he voluntarily submitted himself to trial after he was accused of selling life-saving Bolivian visas to up to 15,000 Jews in Berlin, Warsaw, Kaunas and Stockholm.