The onset of the First World War in 1914 saw "Orient Express" services suspended. They resumed at the end of hostilities in 1918, and in 1919 the opening of the Simplon Tunnel allowed the introduction of a more southerly route via Milan, Venice, and Trieste. The service on this route was known as the Simplon "Orient Express", and it ran in addition to continuing services on the old route. The Treaty of Saint-Germain contained a clause requiring Austria to accept this train: formerly, Austria allowed international services to pass through Austrian territory (which included Trieste at the time) only if they ran via Vienna. The Simplon "Orient Express" soon became the most important rail route between Paris and Istanbul.