Question

How did the Open Door policy help prevent any one nation from monopolizing trade with China?

Answer 1

America intervened in order to get a piece of the action, and to prevent the British or Germans, who invaded first, to completely dominate and control Chinese ports.  

But all it did was to further weaken the Chinese government, and allowed the British to further take advantage of China's weakened state, as well as allowing Japan into China, which set up the stage for Japan's invasion of China prior and in WWI