Russia ignored an Anglo-French ultimatum to withdraw from the Danubian Principalities, the United Kingdom and France had both declared war. Dardanelles Britain was concerned about Russian activity and Sir John Burgoyne, a senior advisor to Lord Aberdeen, urged for the Dardanelles to be occupied and works of sufficient strength to be built to block any Russian move to capture Constantinople and gain access to the Mediterranean. The Corps of Royal Engineers sent men to the Dardanelles, and Burgoyne went to Paris and met with the British ambassador and the French emperor. Lord Cowley wrote on 8 February to Burgoyne, "Your visit to Paris has produced a visible change in the Emperor's views, and he is making every preparation for a land expedition in case the last attempt at negotiation should break down". Burgoyne and his team of engineers inspected and surveyed the Dardanelles area in February. They were fired on by Russian riflemen when they went to Varna. A team of sappers arrived in March, and major building works commenced on a seven-mile line of defence, which was designed to block the Gallipoli Peninsula. French sappers worked on half of the line, which was finished in May. Peace attempts Nicholas felt that because of Russian assistance in suppressing the Hungarian revolution of 1848, Austria would side with him or at the very least remain neutral. Austria, however, felt threatened by the Russian troops in the Balkans. On 27 February 1854, the United Kingdom and France demanded the withdrawal of Russian forces from the principalities. Austria supported them and, without declaring war on Russia, refused to guarantee its neutrality. Russia's rejection of the ultimatum proved to be the justification used by Britain and France to enter the war. Russia soon withdrew its troops from the Danubian Principalities, which were then occupied by Austria for the duration of the war. That removed the original grounds for war, but the British and the French continued with hostilities. Determined to address the Eastern Question by putting an end to the Russian threat to the Ottomans, the allies in August 1854 proposed the "Four Points" for ending the conflict in addition to the Russian withdrawal: Russia was to give up its protectorate over the Danubian Principalities. The Danube was to be opened up to foreign commerce. The Straits Convention of 1841, which allowed only Ottoman and Russian warships in the Black Sea, was to be