had reached the lines of Trajan's Wall, where they were finally halted. In the centre, the Russian forces crossed the Danube and laid siege to Silistra from 14 April with 60,000 troops. The defenders had 15,000 troops and supplies for three months. The siege was lifted on 23 June 1854. The British and the French could not then take the field for lack of equipment. In the west, the Russians were dissuaded from attacking Vidin by the presence of the Austrian forces, which had swollen 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 who were living under Ottoman rule to rebel. When the Russian troops crossed the River Pruth into Moldavia, the Orthodox Christians showed no interest in rising up against the Ottomans. Adding to Nicholas I's worries 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 the Ottomans, the Austrians entered the war on the side of the Ottomans with an attack against the Russians in the Danubian 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 to begin abandoning the principalities. The lifting of the siege reduced the threat of a Russian advance into Bulgaria. In June 1854, the Allied expeditionary force landed at Varna, a city on the Black Sea's western coast, but made little advance from its base there. Karl Marx was noted to have quipped that "there they are, the French doing nothing and the British helping them as fast as possible". In July 1854, the Ottomans, under Omar Pasha, crossed the Danube into Wallachia and on 7 July 1854 engaged the Russians in the city of Giurgiu and conquered it. The capture of Giurgiu by the Ottomans immediately threatened Bucharest in Wallachia with capture by the same Ottoman army. On 1854, Nicholas I, responding to an Austrian ultimatum, ordered the withdrawal of Russian troops from the principalities. Also, in late July 1854, following up on the Russian retreat, the French staged an expedition against the Russian forces still in Dobruja, but it was a failure. By