Could the use of the Flying Tigers be seen as U.S. "choosing sides" before officially entering the war? How?
The United State's use of Flying Tigers can definitely be seen as the United States choosing to stand against the Japanese before officially entering the war in December of 1941. If you remember, there were multiple ways the United States began "choosing a side" in the Second World War prior to entering the war. President Roosevelt pushed for Congress to pass the Lend Lease Act in January 1941. This allowed the US to send or offer defense materials to foreign governments the president felt was necessary for the security of the United States. This same behind the scenes approach was taken in the Pacific Theatre with the Flying Tigers. Recall that Japan invaded Manchuria in the early 1930s and continued their conquest throughout the rest of the decade. At that time, the United States was very concerned with China and began placing trade restrictions on Japan in hopes of pressuring the Japanese to leave China. However, Japan refused to back down and in 1940, after their alliance with the Axis Powers became concrete, invaded Indochina. This action prompted the United States to end all trade with Japan. The following year is when the US began utilizing the Flying Tigers to help aid China keep a flow of vital resources coming in to help prevent total Japanese domination in the country. Not long after this, the United States and Japan held negotiations. Here, Japan demanded restoration of trade and free rule in China whereas the US demanded all Japanese troops out of China. Obviously Japan refused, and these tensions ultimately led to Japan bombing Pearl Harbor on December 7, 1941. The following day the United States declares war on Japan.
This chain of events shows how the United States kept siding with China by exerting pressure on the Japanese through trade restrictions and other strategic moves which aided China in staying afloat. The problem the United States faced was a strong public support for neutrality, especially following WWI. Since the United States had not been directly attacked at that point, the best President Roosevelt could do were different trade restrictions and send aid to China that was not deemed military aid. I hope this helps answer your question!