The starting point for American Imperialism is really post-Civil War, when America wanted to cement its national identity and slide onto the stage as a world power. The places it saw an opening were South and Central America, as well as the Pacific. The U.S. looked to show dominance in these regions.
1866 saw the U.S. Navy open Japan up for trade at the threat of total destruction from bombardment. In 1895, the U.S. annexed Hawaii. America soon took Cuba, Puerto Rico, Guam, and the Philippines from Spain during the Spanish American War in 1898. But, much to the disappointment of locals who thought the United States to be anti-imperialist due to its own history with the British, it did not liberate the people there. Instead, the United States continued to subjugate these people under American rule. The United States economy needed raw materials and labor in order to expand its abilities to export goods. Thus it continued to insert itself into the political affairs of a variety of Latin American countries, such as Venezuela in 1902 and Columbia in 1903, and searched the Pacific Ocean for more islands to claim sovereignty over.
All of America's actions, best characterized by Teddy Roosevelt as "speak soft but carry a big stick," required strong-arm tactics. In 1904, the Roosevelt Corollary claimed the United States had the right to intervene anywhere in the Americas or the Caribbean that needed to be "stabilized". The decades that followed all of these U.S. interventions were filled with blowback and resentment. For example, imperialism in the Philippines, which was annexed following the Spanish American War, led to decades of resistance, bloodshed, and guerrilla warfare. Nicaragua in 1911 and Haiti, which the US invaded in 1915 then occupied through 1934, were other places where rebels and their occupiers clashed. Thousands of rebels, innocent civilians, and U.S. servicemen died in these conflicts. Besides the human cost of lives lost, the monetary cost of maintaining its imperialistic gains proved to be high for the United States.
The most dramatic backlash the United States faced from its imperialism came from Japan. After its ports were forcibly opened by the United States in 1866, Japan quickly militarized and sought to become a power in its own right. These desires led to wars with Russia, Korea, and China in the following decades, before Japan directly invaded United States pacific territories and attacked Pearl Harbor in 1941.
The long term consequences from Imperialism can still be felt today. Many places throughout the Americas saw the pro-United States regimes toppled, such as Cuba, or politically challenged, such as Nicaragua. Intentions of the United States are often viewed skeptically or suspiciously by its neighbors throughout the Americas. The legacy of the Roosevelt Corollary and the size of the United States military still make the possibility of another intervention feel possible. Leaders in countries such as Venezuela make good politics by opposing initiatives of the United States to defend their people from further intrusions.