In the last half of the 19th century, the US’s policy towards China was motivated largely by a desire to get a piece of trade with that country.  During this time, China was dominated by European powers and Japan.  These powers had forced China to grant them a variety of concessions when it came to trade.  The main goal of the United States was to ensure that it was not frozen out of that trade.  This was the point of the “Open Door” policy.  The US policy was to push the European countries to allow China to trade freely with any foreign country.