1. There are a number of islands that are between the United States and China. However, the named ones that would be used would be Hawaii (which is part of the U.S.), followed by Guam (A U.S. territory), and then the usage of Philippines (A U.S. annexed territory), and then the usage of Japan as well (through the usage of open-door policy) as another option for access to China.
2. The United States wants control of these islands to be able to build infrastructure to keep trade ships afloat with resources, as well as refilling ports. On top of that, they can be places to draw in cheaper resources, to raise funds through taxations, and the land could be used to build forward military installations in case of war.
3. American businesses on the East Coast benefitted from the Panama Canal, as it was much more cheaper and faster to go through the Panama Canal which was located in Central America (Panama), instead of going around the Cape Horn, located on the southern most tip of Chile.
