The Spanish-American War was fought in 1899-1900, and made the United States into an empire. The US acquired the Philippines, Puerto Rico and Guam, and ended the colonial era of Spain. But William McKinley was President at that time, and Teddy Roosevelt actually fought in that war.
The US was relatively isolationist before that war, and many people in the country were against our involvement, but once it was over, with a quick victory, the US entered an imperialist decade that was expanded upon by President Roosevelt once he took office in 1901. Roosevelt was key in taking the Panama Canal Zone, establishing American influence in the Caribbean and negotiating peace between Japan and Russia.
While America was isolationist at the start of World War I, it was definitely imperialist under Teddy Roosevelt.