On 28 June 1914, Serbian nationalists assassinated the Archduke Francis Ferdinand. But it was not until 23 July, when the Austro-Hungarian government published its ultimatum to Serbia, that there seemed to be danger of a major war. In response to the news of this ultimatum, Jaurès turned an election speech near Lyons into what was to be his last plea for peace on French soil: ‘We have at this moment against us, against peace, against human life, terrible odds, in the face of which the proletarians of Europe must make every effort at solidarity of which they are capable.’

Saumoneau was one of the first to make that effort. She had already called a meeting of the executive for 22 July, to discuss the measures to be taken in the event of a limited war between Austria and Serbia. The executive authorised an ‘extraordinary meeting’ of the group for the 24th. Faced with the news of the Austrian ultimatum, the GDFS planned a special meeting for 3 August, but events moved too fast for it. On 28 July, Austria declared war on Serbia. Upon learning of this, Saumoneau brought the executive together for an emergency meeting. That same day, 29 July, the leaders of international socialism met at Brussels in a desperate effort to organise a campaign against the war.