How did the Second Sino-Japanese War merge with the larger conflict of World War II?

The Second Sino-Japanese War, beginning in 1937, became an integral part of World War II following the Marco Polo Bridge Incident. This conflict escalated from localized clashes into full-scale warfare between China and Japan. Japan's expansionist ambitions in China, including the occupation of Manchuria and subsequent advances into northern China, directly contributed to the outbreak of this war. The failure of diplomatic efforts and Japan's determination to assert control over East Asia, coupled with China's unified resistance, transformed the regional conflict into a major theater of World War II, particularly after the attack on Pearl Harbor in 1941.123456