The history of American overthrows of foreign governments can be divided into three parts. First came the imperial phase, when Americans deposed regimes more or less openly. None of the men who overthrew the Hawaiian monarchy tried to hide their involvement. The Spanish-American War was fought in full view of the world, and President Taft announced exactly what he was doing when he moved to overthrow the governments of Nicaragua and Honduras. The men who directed these "regime change" operations may not have forthrightly explained why they were acting, but they took responsibility for their acts.
After World War II, with the world political situation infinitely more complex than it had been at the dawn of the century, American presidents found a new way to overthrow foreign governments. They could no longer simply demand that unfriendly foreign leaders accept the reality of American power and step down, nor could they send troops to land on foreign shores without worrying about the consequences. This was because for the first time, there was a force in the world that limited their freedom of action: the Soviet Union. During the Cold War, any direct American intervention risked provoking a reaction from the Soviets, possibly a cataclysmic one. To adjust to this new reality, the United States began using a more subtle technique, the clandestine coup d’état, to depose foreign governments. In Iran, Guatemala, South Vietnam, and Chile, diplomats and intelligence agents replaced generals as the instruments of American intervention.
By the end of the twentieth century, it had become more difficult for Americans to stage coups because foreign leaders had learned how to resist them. Coups had also become unnecessary. The decline and collapse of the Soviet Union and the disappearance of the Red Army meant that there was no longer any military constraint on the United States. That left it free to return to its habit of landing troops on foreign shores.
~ Stephen Kinzer, Introduction to Overthrow: America's Century of Regime Change From Hawaii to Iraq