The Japanese had long since begun the process of modernizing their island nation from a medieval atavistic Samurai nation to a 20th century country. Swords and arrows were replaced with rifles and machine guns, horses were replaced with tanks and signal flags and fires were replaced with radio. At the beginning of the 20th century, the Tsar Alekseyev, son of Tsar Alexander II, was imbued with the urge to expand the Russian borders east. On the 3rd of October, 1903, he sent a letter to the Emperor of Japan. It stated that the Japanese had to recognize Russian interests above the 39th parallel. The Japanese would have none of it. On February 6th, 1904, the Japanese closed their embassy in St. Petersburg, and two days later they evacuated Japanese nationals living there.
Most of the Russian heavy fleet was in port, while four destroyer were out on patrol. On the 9th of February, 1904, Russians on watch thought thye were seeing the profiles of the returning destroyers. It was in fact a Japanese naval srike package. They entered Port Arthur and armed with intelligence of the positions of each Russian ship, they sank two battleships and a cruiser and escaped.
It continued to deteriorate for the Russians. On the 10th of February, Japanese invaded Manchuria grimly determined to take it from the Russians. The Japanese invaded and the Russians dug heavy trenches, a portend perhaps to World War I. The Japanese ground out a deadly war of attrition that resulted in the first modern industrialized war of the 20h century. Port Arthur sin Liaodong Province and it was leased to the Russians. The Tsar of course new that the empire desperately needed a freshwater port other than Sebastopol. Russia had a few fresh water ports that were frozen half the year, but Port Arthur was operational year round.
When the Russo-Japanese War ended, the Japanese had 500,000 fighters, and Russia had 1,000,000 men under arms. The Battle of Yalu River on May 1, 1904 was the first ever Asia victory over a western army in modern history. The war is marked also by the fifth column help the Japanese had against the Russians. The Japanese tended to keep Manchurian towns clean and treated the people fairly. The Russian by contrast hardly mixed with the constituencies that comprised the population of the towns that they had purview. So in a battle of popularity, the Japanese had a plan and it worked. Again, the Japanese were first welcomed as liberators.
The Battle of the Yellow Sea was a victory for Russia and the first ever exchange of fire between heavily armored battleships.
The Battle of Mukden February 20th to Match 10th.
In the Battle of Tsushima Straits on 27–28 May 1905, the Russians lost most of their heavy Baltic Navy in the area between Japan and Korea. It was this battle that forced the Russians to sue for peace. The treaty was negotiated by Theodore Roosevelt. The Russians lost Korea to Japan, they gave Port Arthus back to the Chinese, and Sakhalin Island went to Japan.