then, the Russian withdrawal was complete, except for the fortress towns of northern Dobruja, and Russia's place in the principalities was taken by the Austrians as a neutral peacekeeping force. There was little further action on that front after late 1854, and in September, the allied force boarded ships at Varna to invade Crimea. Black Sea theatre The naval operations of the Crimean War commenced with the dispatch in mid-1853 of the French and the British fleets to the Black Sea region, to support the Ottomans and to dissuade the Russians from encroachment. By June 1853, both fleets had been stationed at Besikas Bay, outside the Dardanelles. With the Russian occupation of the Danube Principalities in July 1853, they moved to the Bosphorus, and on 3 January 1854, they entered the Black Sea. Meanwhile, the Russian Black Sea Fleet operated against Ottoman coastal traffic between Constantinople and the Caucasus ports, and the Ottoman fleet sought to protect the supply line. The clash came on 30 November 1853, when a Russian fleet attacked an Ottoman force in the harbour at Sinop and destroyed it at the Battle of Sinop. The battle outraged British public opinion, which called for war. There was little additional naval action until March 1854, when after the declaration of war, the British frigate was fired on outside Odessa Harbour. In response an Anglo-French fleet bombarded the port and caused much damage to the town. To show support for the Ottomans after the Battle of Sinop, on 22 December 1853, the Anglo-French squadron entered the Black Sea and the steamship HMS Retribution approached the Port of Sevastopol. Its commander received an ultimatum not to allow any ships in the Black Sea. In June, the fleets transported the Allied expeditionary forces to Varna to support the Ottoman operations on the Danube. In September they again transported the armies, this time to Crimea. The Russian fleet then declined to engage the allies but preferred to maintain a "fleet in being", a strategy that failed when Sevastopol, the main port and the base of most of the Black Sea fleet, came under siege. The Russians were reduced to scuttling their warships as blockships after they had stripped them of their guns and men to reinforce batteries on shore. During the siege, the Russians lost four 110- or 120-gun, three-decker ships of the line, twelve 84-gun two-deckers and four 60-gun frigates in