In August 1964, a small military engagement off the coast of North Vietnam helped escalate the involvement of the United States in Vietnam; the Vietnam War would become the longest military engagement in American history prior to the Iraq and Afghanistan wars.
In 1899 and 1900, Secretary of State John Hay issued what became known as the Open Door Notes to foreign powers involved in China. Secretary Hay called on those powers to respect the rights of each other, to agree to an open market and equal trading opportunities for merchants of all nationalities, and to respect the territorial and administrative integrity of China.
The treaty of peace ending the Spanish-American War resulted in the United States obtaining the Philippine Islands from Spain. Despite intense political opposition to the acquisition of the islands, the U.S. Senate ratified the treaty. The political impact of anti-imperialist arguments, the difficult experience of suppressing native Filipino resistance, and the lack of attractive opportunities for further territorial expansion, all effectively stalled the American imperialist/expansionist movement.