British and French armed forces disembarked at Varna, supported by the newly-organized Allied Fleet: nineteen ships of the line, consisting of fifteen sailing vessels and four steam-powered ships of the line as well as numerous steam-frigates. Nicholas I and Admiral Menshikov refused even to consider advancing against such a powerful enemy on the open sea. Thus the Black Sea Fleet was concentrated at Sevastopol under Vice-Admiral Kornilov. Admiral Menshikov, responsible for defending the Crimea, did not foresee an Allied invasion and prepared neither Sevastopol nor the region as a whole for such a possibility. Both Menshikov and the Emperor were convinced that, after the Russian withdrawal from the Danube, the British and French would agree to peace negotiations. This miscalculation was to have serious repercussions.

On 10 April 1854, Allied vessels appeared near Odessa, and the town immediately came under fire. On 30 April the 16-gun British steamer frigate Tiger went aground in a fog near Odessa. After being bombarded by Russian artillery, the Tiger surrendered and 226 sailors were taken prisoner.

In early September the entire Allied fleet, consisting of 89 combat vessels and 300 transports, sailed up the Crimean coast and landed an army of 62,000 troops at Evpatoria. After routing Menshikov's comparatively small force at the Battle of Alma, the Allies marched on Sevastopol. The prospects for Sevastopol were bleak because the city had neither ground forces nor an adequate garrison to protect its port. Therefore, Vice-Admiral Kornilov argued that Sevastopol must be defended by engaging the approaching enemy on the open sea.

The Russian forces could, at the very least, cut the English and French ships off from their supply lines. However, Admiral Menshikov and the majority of the commanders disagreed with Kornilov and determined that the better tactic was to secure and defend the town and its port as best as possible.

Menshikov ordered that part of the fleet be sacrificed and sunk at the entrance to the North Bight and their crews and guns be used ashore for the defence of the fortress. Menshikov himself left Sevastopol with the surviving regiments, having ordered Kornilov and the other admirals to fortify the town's defences. On 5 October the enemy approached Sevastopol but were forced back by Russian troops. On the same day fourteen French, two British and two Turkish ships of the line, armed with a total of 1300 cannon, drew near the fortress and attacked its seaside forts, protected by 275 cannon. After a six-hour struggle, the Russians gained the upper hand, and the Allied fleet retreated after suffering 350 casualties. Bombardment of the fortress continued for several days, claiming the life of Vice-Admiral Kornilov among other defenders.

From October 1854 to March 1855, the British and French continued to besiege and bombard the town, making periodic attempts to invade the fortress. The defenders answered with counterattacks at night. Admiral Nakhimov, who had by this time become a national hero, rallied the city's defenders. He was not alone in his heroism. Lieutenants Lev Batyanov, Nikolay Birulev, Nikolay Astapov and Pyotr Zavalishin also earned distinction for their efforts.

At times nature itself aided the defenders of Sevastopol. In October 1854, a Turkish ship of the line and a frigate were wrecked in a storm off the coast of Rumeli. On 2 November a ferocious hurricane sank the 100-gun French ship of the line Henri IV as well as more than a dozen other vessels and large steamer-transports. However, after the hurricane abated, nothing prevented the Allies from resupplying their forces. The Russian army was, on the other hand, unable to break through the Allied troops surrounding the fortress. Failure followed success, and, shortly after their victory at Balaklava, the Russians lost the Battle of Inkerman. During the night of 13 February 1855, three more ships of the line and two frigates were purposely sunk by the Russians at the entrance to the North Bight. In late March the bombardment of Sevastopol increased in intensity, accompanied by attacks on the fortifications. On 27 August the Allied forces began to storm the town.

The Russian sailors continued to hold their positions, but their ranks were growing steadily thinner. On 7 March Rear Admiral Vladimir Istomin was killed on Malakhov Hill, and on 28 June Admiral Nakhimov was mortally wounded. The once prosperous town was in ruins. On 28 August, after the loss of Malakhov Hill, the remaining Russian officers decided to abandon Sevastopol. On the night of 29 August the defenders crossed over a pontoon bridge on the northern side of the bight. As they left, they blew up the town's batteries and powder-magazines so that the supplies in the fortress could not be used by the enemy. In addition, all remaining vessels of the Black Sea Fleet were sunk in the inner harbour.

Russian losses were very high. The entire Black Sea Fleet was annihilated; three admirals, 106 officers and 3,777 sailors were killed; nearly 14,000 seamen and officers were wounded. In spite of the catastrophic defeat at Seva-stopol, the meritorious effort to defend the fortress is regarded by historians as one of the most distinguished moments in Russia's mi-litary history. Fifty-eight of the naval officers who had defended the fortress were awarded the Order of St. George and seventeen fleet equipages received the St. George banner bearing the inscription, "For standing fast at Sevastopol from 13 September 1854 to 27 August 1855."