Question

Who were the Chinese constantly fighting with?

Answer 1

Japan.

Chinese and Japanese fought against each other in four periods:

1. 660s CE: on Korean Peninsula during which Tang dynasty scored a complete victory;
2. 1590s CE: on Korean Peninsula (again): during which Ming forces defended Koreans from marauding Japanese invaders;
3. 1894–94: during which Imperial Japan inflicted humiliating and crushing defeat against the corrupt and decaying Qing dynasty China;
4. 1931–45: China fought against the genocidal invading Japanese Imperial Armed Forces.

Answer 2

Japan

Some people might say that the answer is US, but in WW2, US helped China. Even in other times of hardship, their friendship can be proved.

Step-by-step explanation:

On that date, outside of Beijing, Japanese and Chinese troops clashed, and within a few days, the local conflict had escalated to a full, though undeclared, the war between China and Japan. The war between China and Japan was at first a conflict in which no western powers were openly involved. The Second Sino-Japanese War, (1937–45), was a conflict that broke out when China began a full-scale resistance to the expansion of Japanese influence in its territory (which had begun in 1931). Conflict in Asia began well before the official start of World War II. Seeking raw materials to fuel its growing industries, Japan invaded the Chinese province of Manchuria in 1931. By 1937 Japan controlled large sections of China, and war crimes against the Chinese became commonplace.