The event that provoked Pres. Roosevelt to sell weapons to China occurred in 1937. This was when Japan launched a full-scale war with China.
Before July of 1937, Japan had been occupying Manchuria, a region in Northeast China. However, Japan had not yet actually invaded China itself. In July 1937, however, the Japanese started a war in which they invaded China. At this point, Roosevelt decided that something had to be done. Roosevelt announced that weapons could be sold to China. He ruled that the Neutrality Act did not apply because China and Japan were not technically at war. This meant that it would be okay to sell weapons.
So, the event was the 1937 attack on China by Japan.