What is the open door policy?

What is 'Open Door Expansion/Policy' ?

• I'm reading a book called "The Tragedy of American Diplomacy" by William Appleman Williams and he talks about 'Open Door Expansion' or the "Open Door Policy" a lot but doesn't really define it. He says "It was the decision of the United States to employ its new and awesome power in keeping with the traditional Open Door Policy which crystalised the cold war" What does he mean by this?

• Answer:

  The Open Door policy was American policy to prevent any one country dominating a particular area or country and thus closing it off from US exports.It was originally applied to China beginning in the late 19th century.