During the period 1880–1910, two new phenomena appeared that were to have great significance. The first was American participation in the vast collective treaties negotiated in Europe that dealt with such matters as the rules of warfare at sea and other issues important to American overseas commercial activities. Whereas the United States had been absent from the congresses of Vienna (1814–1815), of Paris (1856), and of Berlin (1878), as well as from the ambassadorial conferences of the Concert of Europe (an obvious consequence of the Monroe Doctrine), in 1880 it participated in the Madrid Conference on Morocco (treaty approved by the Senate on 5 May 1881) and in 1884 and 1885 in the Berlin Conference on equatorial Africa. In July 1890 the United States signed an international agreement on the suppression of the slave trade in Africa (approved by the Senate on 11 January 1892).
The United States also participated in the 1888–1889 Pan-American Conference in Washington, D.C., which created the Pan American Union; it had refused to take part in the attempt to create such a union in 1826. Later it took a seat at the Pan-American conferences in Mexico City (1901–1902), Rio de Janeiro (1906), and Buenos Aires (1910), as well as at the First International Peace Conference at The Hague (1899), which created the Permanent Court of Arbitration, and at the Second International Peace Conference at The Hague (1907). The United States also was represented at the Algeciras Conference (1906) on Morocco. On this occasion the Senate accepted the resulting agreement (in December 1906), but with the reservation that its approval did not signify a break with the traditional policy of noninvolvement in Europe.
The second new development was that for the first time in its history, the United States assumed the role of mediator between two great powers, Russia and Japan, then at war. President Theodore Roosevelt had been anxious to maintain the balance of power in the Far East. He knew that the United States possessed an effective means of putting pressure on the victorious Japanese, that is, the possibility of helping them obtain funds to revive their bankrupt economy. Consequently, as early as April 1905, before the decisive Russian naval defeat at Tsushima, he secretly agreed to serve as a mediator. Japan accepted mediation on 31 May, after the battle of Tsushima, for it realized that it was better to limit its ambitions than to risk provoking a coalition against it. The resulting Treaty of Portsmouth, in part a product of American initiatives, dissatisfied Japanese ultranationalists. The United States also was mediator (without being a signatory) at the Central American Peace Conference held in November–December 1907 in Washington, D.C.