As a result of his foreign language skills, Müller was the representative of the SPD at the Second International and at the conventions of socialist parties in other countries in western Europe. In late July 1914, Müller was sent to Paris to negotiate with the French socialists over a common stance towards the respective countries' war loan proposals. No agreement was reached, however, and before Müller was able to report back, the SPD had already decided to support the first war loans in the Reichstag.