The Russo-Japanese War (1904–1906) was a conflict that grew out of the rival imperialist ambitions of Russia and Japan in Manchuria and Korea. The major theatres of the war were Port Arthur, Vladivostok and the Liaodong Peninsula, plus up the railway from the port to Harbin. Also the major revolts in Poland and Finland are considered part of the war due to the effects they caused to the Russian war effort. The Russians were in constant pursuit of a warm water port. The Japanese were driven to war through geostrategic concerns to secure their interior lines by stemming Russian interest in Korea.

In the late 19th century and early 20th century, various Western countries were competing for influence, trade, and territory in East Asia while Japan strove to transform herself into a modern great power. Great power status at the time depended in part on access to colonies which could provide raw materials. Securing colonies in turn depended on naval power, which required bases for the increasingly large battleships of the era, and a chain of coal stations for warships to restock the fuel for their boilers.

The Japanese government recognized Korea as the lifeline of Japan since Korea is geopolitically close to Japan. Also, in 13th century, Japan was attacked by the Yuan dynasty of Mongolia which passed through the Korean peninsula. Korea was traditionally subordinated to China. At first, the Japanese government wished to part Korea from China, and form Korea into an independent country, then try to make an alliance. However, this did not work since China strongly stated their sovereignty over Korea.

There were several conflicts, but finally it became the Sino-Japanese War. Japan's subsequent defeat of China led to the Treaty of Shimonoseki (17 April 1895), under which China abandoned its own suzerainty to Korea and ceded Taiwan and Lüshunkou (often called Port Arthur). However, three Western powers (Russia, the German Empire and the French Third Republic), by the Triple Intervention of 23 April 1895 applied pressure on Japan to relinquish Port Arthur. The Russians later (in 1898) negotiated a 25-year lease of the naval base with China, and sent soldiers. Meanwhile, Japanese forces were trying to take over Korea, which had a protection pact with Russia. Russian forces consequently occupied most of Manchuria and northern parts of Korea.

Hirobumi Ito started to negotiate with Russia for exchanging Manchuria and Korea. He knew Japan didn't have enough power to fight with Russia, so he thought if Japan admitted Russian control over Manchuria, then, Japan will keep Korea with negotiating with Russia. However, Japan and the U.K made alliance in 1902 since the U.K didn't wish Russia's advancing toward south. Therefore, Ito couldn't get many supporters.

After failing to negotiate a favorable agreement with Russia, Japan sent an ultimatum on 31 December 1903 and severed diplomatic relations on 12 January 1904. Three hours prior to the ultimatum being received by the Russian Government, Japan attacked the Russian Navy at Port Arthur. Both sides issued a declaration of war on 16 January. Under international law, Japan's attack was not considered a surprise attack, because of the ultimatum.

Port Arthur, on the Liaodong Peninsula in the south of Manchuria, had been fortified into a major naval base by the Russians. Needing to control the sea in order to fight a war on the Asian mainland, Japan's first military objective was to neutralize the Russian fleet at Port Arthur. On the night of 12 February 1904, the Japanese fleet under Admiral Heihachiro Togo opened the war with a surprise torpedo attack on the Russian ships at Port Arthur, badly damaging two battleships. The attacks developed into the Battle of Port Arthur the next morning. A series of indecisive naval engagements followed, in which the Admiral Togo was unable to attack the Russian fleet successfully under the land guns of the harbor and the Russians declined to leave the harbor for the open seas, especially after the death of Admiral Wilgelm Vitgeft on 13 April.

However, these engagements provided cover for a Japanese landing near Inchon in Korea, from which they occupied Seoul and then the rest of Korea. By the end of April, the Japanese army under Kuroki Itei was prepared to cross the Yalu river into Russian-occupied Manchuria.

In counterpoint to the Japanese strategy of gaining rapid victories to control Manchuria, Russian strategy focused on fighting delaying actions to gain time for reinforcements to arrive via the long Trans-Siberian railway. On 1 May 1904, the Battle of the Yalu River, in which Japanese troops stormed a Russian position after an unopposed crossing of the river, was the first major land battle of the war. Japanese troops proceeded to land at several points on the Manchurian coast, and in a series of engagements drove the Russians back on Port Arthur. These battles were marked by heavy Japanese losses attacking entrenched Russian positions, but the Russians remained passive and failed to counterattack.

At sea, the war was just as brutal. After the 12 January attack on Port Arthur, the Japanese attempted to deny the Russians use of the port. During the night of 18-20 January, the Japanese attempted to block the entrance to Port Arthur by sinking several cement-filled steamers in the deep water channel to the port, but they sank too deep to be effective. Another attempt to block the harbor entrance during the night of 3-4 April with blockships also failed.

By then, both sides were engaged in a tactical offensive, laying mines in each other's ports. This was the first time that mines were used for offensive purposes; in the past, mines had been used for purely defensive purposes to protect harbors against potential invaders. The Japanese mine-laying policy proved effective at restricting the movement of Russian ships outside Port Arthur, when on 13 April 1904 two Russian battleships, the flagship Petropavlovsk and the Pobeda, struck Japanese mines off Port Arthur. The Petropavlosk sank within an hour, while the Pobeda had to be towed back to Port Arthur for extensive repairs. Admiral Vitgeft died on the Petropavlovsk by choosing to go down with his ship.

The Russians soon copied the Japanese policy of offensive minelaying. On 15 May 1904, two Japanese battleships were lured into a recently laid Russian minefield off Port Arthur, each striking at least two mines and sinking later on that day. On 18 July, a breakout attempt by the Russian squadron failed. By the end of the month, Japanese artillery were already putting shells into the harbor.

Japan began a long siege of Port Arthur, which had been heavily fortified by the Russians. On 19 September 1904, the Russian fleet attempted to break out and proceed to Vladivostok, but they were intercepted and defeated at the Battle of the Yellow Sea. The remnants of the Russian fleet remained in Port Arthur, where they were eventually sunk by the artillery of the besieging army. Attempts to relieve the city by land also failed, and after the Battle of Liaoyang in late September the Russians retreated to Mukden (Shenyang). Port Arthur finally fell on December 25 1904, after a series of brutal, high-casualty assaults.

The Japanese army was now able to attack northward. To finish the war, Japan needed to crush the Russian army in Manchuria. The Battle of Mukden commenced at the end of February. Japanese forces progressed step by step and tried to encircle General Kuropatkin Headquarters at Mukden (Shenyang). Russian forces resisted and on 10 March 1905 the Japanese decided to retreat. Having suffered massive casualties, the Japanese decided to abandon the field. Because strategically the possession of the city meant little, a final victory would depend on the navy.

Meanwhile, at sea, the Russians had already been preparing to reinforce their fleet the previous year by sending the Baltic Sea fleet under Admiral Stepan Osipovich Makarov around the Cape of Good Hope to Asia. On 8 October 1904, while passing by the German coast they nearly provoked a war by firing on German fishing boats that they mistook for torpedo boats.Units of the High Seas Fleet responded and a full blown naval battle was barely averted.

The long duration of its journey meant that Admiral Togo was well aware of the Baltic Fleet's progress, and he made plans to meet it before it could reach Vladivostok. He was planning to intercept them in La Perouse Strait between Korea and Japan but he arrived an hour too late, by the time he arrived to the Strait on March 19 1905 the Russian Fleet escaped thru the fog.

With the Navy's failure to apply the expected knockout punch to the Russian Navy, it was up to the Japanese army to try to bring Russia to the negotiation table. The city of Vladivostok was marked for capture by the Japanese Army and forces began to be deployed northward around the city by late June starting the Battle of Vladivostok. On July 20 1905, Admiral Makarov died during a Japanese artillery bombardment and the morale of the Russian defenders plummeted with mutinies occuring in some instances. But to finish the war, Japan needed to crush the Russian army in Manchuria. The 2nd Battle of Mukden commenced on August 19 1905, the same day Vladivostok fell to Japanese troops. Japanese forces progressed step by step and and again tried to encircle General Kuropatkin Headquarters at Mukden (Shenyang). Again the Russian forces resisted but this time on September 3 1905 the Russian defenses were broken and defenders routed north. Having suffered massive casualties, the Japanese did not pursue the Russians.

Although Russia still had a larger army than Japan, these successive defeats coupled the chaos caused by the large scale rebellions in Western Russia had shaken Russian confidence. Throughout 1905, Russia was rocked by the Finnish and Polish Revolutions of 1905, which posed a severe threat to the stability of the government. Russia elected to negotiate peace rather than continue the war, so that it could concentrate on internal matters.

An offer of mediation by U.S. President Elihu Root (who earned a Nobel Peace Prize for this effort) led to the Treaty of Washington, signed on 12 January 1906. Russia ceded Sakhalin Island and the area around Vladivostok,including the city, to Japan. Russia also signed over its 25-year leasehold rights to Port Arthur, including the excellent naval base and the peninsula around it. Russia further agreed to evacuate Manchuria and recognize Korea and Manchuria as part of the Japanese sphere of influence. Japan would later annex Korea and Manchuria with scant protest from other powers.

This was the first major victory in the modern era of an Asian country over a Western one and a harbinger of a future series of events that would lead to decolonization. Japan's prestige rose greatly as it began to be considered a modern Great Power. Concurrently, Russia lost virtually its entire Eastern and Baltic fleets and slipped in international esteem. This was particularly true in the eyes of Germany. Russia was France's ally, and that loss of prestige would have a significant effect on German plans concerning a potential future war with France.