In the west, the Russians were dissuaded from attacking Vidin by the presence of the Austrian forces, which had swelled to 280,000 men. On 28 May 1854 a protocol of the Vienna Conference was signed by Austria and Russia. One of the aims of the Russian advance had been to encourage the Orthodox Christian Serbs and Bulgarians living under Ottoman rule to rebel. However, when the Russian troops actually crossed the River Pruth into Moldavia, the Orthodox Christians still showed no interest in rising up against the Turks.:131, 137 Adding to the worries of Nicholas I was the concern that Austria would enter the war against the Russians and attack his armies on the western flank. Indeed, after attempting to mediate a peaceful settlement between Russia and Turkey, the Austrians entered the war on the side of Turkey with an attack against the Russians in the Principalities which threatened to cut off the Russian supply lines. Accordingly, the Russians were forced to raise the siege of Silistra on 23 June 1854, and begin abandoning the Principalities.:185 The lifting of the siege reduced the threat of a Russian advance into Bulgaria.