“Intervention is not now, never was, and never will be a set policy of the United States.” Herbert Hoover’s claim was cleverly stated: Even dozens of interventions might be defended as ad hoc responses to particular situations rather than part of a “set policy.”
Still, Latin Americans had reason to be skeptical of Hoover’s assessment. As Niall Ferguson tells the story in Colossus: The Rise and Fall of the American Empire 
And it was not an inadvertent habit. Theodore Roosevelt and Woodrow Wilson combined to supplement the Monroe Doctrine with more aggressive and wide-ranging policies, befitting America’s growth in power since the Monroe administration. It was, Hoover notwithstanding, American policy.
Roosevelt declared in December 1904 that “Chronic wrongdoing, or an impotent which results in a general loosening of the ties of civilized society, may in America, as elsewhere, ultimately require intervention by some civilized nation, and in the Western Hemisphere, the adherence of the United States to the Monroe Doctrine may force the United States, however reluctantly, in flagrant cases of wrongdoing or impotence, to the exercise of an international police power.” Wilson’s policy was more ambitious: He said that the US would not cooperate with any Latin American government that did not meet American standards of “just government based on law.” Military dictators and revolutionaries were out.
Ferguson cites the following conversation between the American ambassador to the UK, Walter Page, and the British foreign secretary, Edward Gray, in 1913. They are discussing Wilson’s decision not to recognize the Mexican government of General Victoriano Huerto, who took power after an assassination:
GREY: Suppose you have to intervene, what then?
PAGE: Make ‘em vote and live by their decisions.
GREY: But suppose they will not so love?
PAGE: We’ll go in and make ‘em vote again.
GREY: And keep this up 200 years?
PAGE: Yes. The United States will be here for two hundred years and it can continue to shoot men for that little space till they learn to vote and to rule themselves.
Thus it was that Marines and the US Navy were sent to support secessionists in Panama in 1903 “to establish Panama as an independent state after the Colombian Senate refused to ratify an agreement leasing land for the construction of the canal.” Thus it was that the Cuban Constitution of 1902 included the Platt Amendment (from Senator Orville Platt), which gave the US the right to intervene “for the preservation of Cuban independence, the maintenance of a government adequate for the protection of life, property and individual liberty” and which also prohibited any bilateral ties between Cuba and a foreign power, giving the US “an effective veto power over the island’s foreign policy.” Thus it was that the US sent troops to Cuba in 1906 to prevent a coup and establish an American governor-general, troops again a few years later, and again in 1912, and 1917-1922. Thus it was that the US took 55% of customs revenues from the Dominican Republic for debt service. Thus it was that “between 1900 and 1913 the United States dispatched small detachments of troops no fewer than sixteen times” to Haiti. Thus it was that the US sent troops into Nicaragua in 1927 to fight guerillas under Augusto Cesar Sandino, and were still there until 1933, leaving Somoza in charge (until 1979).
The results, Ferguson says, were dispiriting: “it is difficult to avoid the conclusion that annexation might have been better for all these places . . . . There was only one true democracy in the entire region by 1939, and that was Costa Rica, where the United States had never intervened.