Japans naval power was well over double the power than Russia, which gave Japan an advantage.
Also when Japan was under the power of Emperor Meiji it had embarked upon a series of sweeping reforms aimed at rapid modernisation along western line. And the Japanese army and navy were far better prepared and equipped than the Russian forces and won a series of major victories.
The reason why the Russian forces were not successful against Japanese was not because their troops fought badly, but their military commanders had not prepared effectively and this allowed the Japanese to out manoeuvre the Russians unimaginative strategy.
In march, 1904 the first Japanese army landed in Korea and Namp’o, it then face the Russians on Yalu river and the Russian commander decided to hold the defensive until reinforcements arrived for them to move offensive.
The Japanese commander built up a 4-1 troop advantage and forced a passage across the Yalu on May 1. The Japanese losses were about 1,100 out of 40,000 men and the Russians lost some 2,300 out of 7,000 troops.
During May the Japanese second army landed on the Liaodong peninsula, and severed communications between Port Arthur and Manchuria and then the Japanese won the battle of Nanshan (May 30) and besieged Port Arthur. The Japanese third army landed on the eastern coast of Korea and then the fourth army landed at Gushan on the Manchurian coast. The Russian forces moved to Liaoyang and the Japanese defeated Russian force of 25,000 at Wafangdian on June 14, the Japanese began to converge on Liaoyang.
Between august 25 and September 4, the battle of Liaoyang was fought and although the Japanese were outnumbered 130,000 to 180,000 they defeated the Russians and they retreated towards Shenyang.
The Japanese were successful but their reserves of trained men were running out and the Russians army was being reinforced by about 30,000 men a month by the trans-siberian railway and the commander decided it was a good time to attack. They launched an attack on the Sha He, which lasted from October 5 to October 17 and then a second attack was launched at Sandepu on January 26-27, 1905 but both broke down.
The Russo Japanese war ended on September 5, 1905. After Port Arthur and the defeats at Shenyang and Tsushima, the emperor finally decided to accept the offer of mediation by US president Roosevelt. The Japanese were ready to agree as they were finacially exhausted but victorious and the treaty of Portsmouth was signed. The Russians surrendered and agreed to leave the lease of Liaoyang and Port Arthur, the southern half was ceded to Sakhalin, and the Russians evacuated Manchuria, and recognised Korea as a Japanese sphere of influence.
There were a couple of things, which happened because of the war. It caused the Russian revolution two months after the treaty of Portsmouth in 1905 and the ever-increasing number of emigration from Russia’s hostile ground.