In 1937 the Japanese Empire invaded China and started the 2nd Sino-Japanese war. They had previously overran Manchuria and now conquered the eastern parts of China, which meant control of of all the Chinese ports. The Japanese were also, during the campaign, guilty of murdering hundreds of thousands of Chinese in one atrocity after another as they overran many of the major cities of China.
President Roosevelt demanded that the Japanese stop their aggression and withdraw from China and instituted an embargo of all American materials going to Japan when they didn't comply. The embargo hurt the Japanese war effort significantly because they depended on American steel and oil to fuel their war machine. China was in dire straights as she had no significant industrial capacity to manufacture the tools of war and had to import them. She desperately needed help to stave off the Japanese. In the early stages of the war with Japan, the Soviets supplied planes and other aid to the Chinese, but was withdrawn when the Soviets got in a war with Germany. The only path open to provide aid to the Chinese that was capable of supplying large amounts of aid was the Burma Road. With no effective air power to defend the road the Japanese were free to bomb Chinese cities and transportation at will. They could make the Burma Road unusable.
Roosevelt undertook aid to China on a rather large scale, supplying weapons and other necessary supplies. Because the Japanese controlled all the Chinese ports, the only way to keep the supplies coming into China was through the Burma Road and through air support by planes flying the hump.
Clair Chennault, at the time, Chief of Staff to Chiang Kai-shek went to Washington and appealed to the President for airplanes and men to help defend the Road and the cities of western China. Of course, because we were not at war with Japan the US couldn't act overtly, so they came up with another plan. Roosevelt would arrange a loan to the Chinese so that they could purchase fighters from American manufacturers and then diverted 100 P40's to China that had been scheduled to be delivered the the RAF. Also, an agreement was reached that let Chennault recruit US pilots to fly in a newly formed American Volunteer Group (AVG) which would become part of the Chinese Air Force to fly the planes. Upon completion of their contract with the AVG they would be allowed to return to their previous service with no loss in rank or seniority.
American help and the exploits of the AVG, which became famous as the Flying Tigers, kept the Burma road open, protected many of her cities and helped keep China from defeat until America entered the war following Pearl Harbor.
Now the question: What would Obama have done? Obama's actions in the Ukraine and in the middle east shows clearly that we would not have become involved directly in the Sino-Japanese war. Obama would have protested, tried diplomacy and threats to try to get the Japanese to behave, but he never would have activity supported China with arms and especially the creation of the only effective air force the Chinese had to protect their western cities and especially the life line into China before Americans entry into the war. Assuming that the Japanese still felt that they had to attack the US. There was no other country with resources that could come to China's aid that wasn't already involved in a war of their own.
Would China have fallen to the Japanese? Without American help, read that Roosevelt's help, it is likely that the whole of China, at least the major parts, would come under Japanese control with their vast resources of food and manpower. That would have set the Japanese up in a much more powerful position. They would use China much as they used Manchuria, moving a lot of their industrial capacity to China and having huge pools of manpower to make the tools of war and feed their troops and Navy in their expanding empire. A quick look at the map shows that from China a nation can dominate the western pacific.
Without the need to station whole armies in China in a combat roll and supplies of men and supplies needed to fight on that front, the Japanese would have far more resources to expand and hold the gains in the pacific. Things would get a lot tougher.
