In 1917, as the War was dragging on towards its fourth year, Sixtus' brother-in-law, Emperor Charles I, secretly entered into peace negotiations with France using Sixtus as intermediary.
Godine 1917., dok se rat primicao svojoj četvrtoj godini, Sixtusov zet, car Karlo I., potajno je ušao u mirovne pregovore s Francuskom, koristeći Sixtusa kao posrednika.