How did U.S. policy affect Latin America's development?

United States policy in Latin America has been more one of interference than anything else; a prime example of the road to hell being paved with good intentions.

Simon Bolivar had envisioned something of a United States of South America which he would call Gran Colombia. Competing interests prevented this from happening, and has caused the only area in the world with separate states sharing the same language and culture.

U.S. policy began with the Monroe Doctrine, in which President Monroe declared the Americas closed to foreign colonization. His real intent was to prevent European countries sending troops to Latin America to protect their financial interests. This was too close to America's borders for comfort. When Theodore Roosevelt promoted the digging of the Panama Canal, he openly supported a revolt of the people of Panama against Colombia at a time when Panama was a part of Colombia. The resulting diplomatic fiasco caused Colombia to break diplomatic relations with the U.S.

Roosevelt's successor, William Howard Taft promoted Dollar Diplomacy, large U.S. loans to attempt to stabilize Latin American governments which have historically been unstable. His successor, Woodrow Wilson, who had an "I"m right, everyone else is wrong" attitude, practiced a policy known as "Missionary Diplomacy." He interfered in a Mexican coup d'etat; and sent American troops under General Pershing into Mexico in a futile attempt to capture Pancho Villa. He also sent American troops into Nicaragua, Haiti and the Dominican Republic to preserve stability there. He commented once to William Jennings Bryan, his Secretary of State:

I suppose there is nothing to do but take the bull by the horns and restore order.

President Ronald Reagan sent troops into Granada, and President George W. Bush also arranged for the capture and arrest of Panamanian strongman Manuel Noriega.

So the U.S. has intervened (or perhaps interfered) in Latin America frequently, when it suited its purpose to do so.

How did US policy influence Latin America's development?

This is a topic that can be debated at length.  People take different sides on this issue depending on their political ideologies.

People on the left of the political spectrum tend to argue that the US hurt Latin America's prospects for development.  They propound a set of ideas known as "dependency theory."  This theory holds that countries like the US essentially use less developed countries like those of Latin America as colonies.  The theory argues that the US keeps these countries down for its own benefit.

Thus, to people who believe in this theory, US policies affected Latin America's development by working to make it difficult for those countries to develop.  US policies kept them in a subordinate position.