The Treaty of Portsmouth marked the end of the Russo-Japanese War, a significant military conflict that lasted from 1904 to 1905, pitting the Russian Empire against the Empire of Japan. The war arose from the rivalry between the two nations over control of Korea and parts of Manchuria, with Japan emerging as a rising power in East Asia. The conflict witnessed several notable battles and highlighted Japan's growing military capabilities, culminating in a significant victory over Russian forces at the Battle of Tsushima. As the war dragged on, both nations faced financial strain and public pressure to seek a resolution.

In the summer of 1905, U.S. President Theodore Roosevelt offered to mediate peace negotiations, a move that was emblematic of the United States' emerging role on the global stage. The negotiations were held at the Portsmouth Naval Shipyard in New Hampshire, where representatives from both countries engaged in discussions from August to September. Roosevelt's involvement was crucial, as he aimed to facilitate a fair settlement that recognized Japan's interests while protecting Russia's dignity.

The treaty was signed on September 5, 1905, and it included several key provisions. Japan was recognized as the predominant power in Korea and received significant territorial concessions in Manchuria, including Port Arthur and the southern half of Sakhalin Island. The Treaty of Portsmouth was significant as it not only acknowledged Japan's rise as a global power but also marked the first time an Asian nation had defeated a European power in modern history. The diplomatic success of Roosevelt further enhanced the United States' reputation and paved the way for future involvement in international affairs.