The history of the United States is the same as in OTL until 1865, when William Seward was assassinated along with Abraham Lincoln. Initially, this had little significant change. However, in 1868, the divergence becomes larger when the United States fails to purchase Alaska from Russia.
In 1893, a US-backed coup overthrew the government of Queen Liliuokalani in Hawaii, establishing a Republic of Hawaii. In 1898, the Spanish-American War began. After the war was over, there was talk of annexing the Philippines. An organization known as the American Anti-Imperialist League was formed. Unlike in OTL, the Anti-Imperialists succeeded in defeating annexation of the Philippines, which were declared a sovereign state. However, the United States did negotiate a treaty to lease certain ports indefinitely, much as had been done with Guantanamo in Cuba.
In 1900, the Anti-Imperialist League endorsed the Democratic candidate William Jennings Bryan for President. He won in a narrowly-contested election, 227 electoral votes to McKinley's 220.