In December 1941, before the attacks on the US and the British Empire, Japan attacked which other country?

The spectacular attack by the Empire of Japan on US naval forces at Pearl Harbor is the first connection that many people make with the date 7 December 1941. Other events at that time may not be so readily recalled. About an hour before the Pearl Harbor assault, the Empire of Japan had launched an attack on Malaya; later the same morning they attacked Hong Kong (both these locations were at that time part of the British Empire). Japan announced declarations of war on the United States and the British Empire the same day (8 December in Japan), but after their surprise attacks.

Before this Japan had started aggression against neutral Thailand. About two hours before the attack on Malaya, the Japanese presented the Thai government with an ultimatum to allow the Japanese military to enter Thailand. The Thais were given two hours to respond; when the Japanese received no response, they invaded. Although there was some determined resistance in Southern Thailand, the fighting lasted only five hours. After the ceasefire Thailand and Japan formed an alliance, making Thailand part of the Axis powers until the end of World War II.

The Republic of China had been in a state of undeclared war with Japan since 1937 and Japan had occupied much of north and coastal China; but there was no new attack in December 1941. Korea had been under Japanese rule since 1910. Japan had been in a border conflict with Mongolia in 1939, which it lost; no action was resumed in December 1941.