When it says that "The United States found itself fighting a war on two fronts" it means that the US was involved in a war where they were forced to fight in two different locations. For example, the United States faced a two front war during World War II. The US was forced to fight in the Pacific Ocean against the Japanese Empire while also fighting against Germany in Europe.
A two front war makes it much more difficult for a country to have success, as they need to split up their resources between two different places instead of having them concentrated in one area.