Although China has the biggest military and the highest population, they won’t necessarily stand a chance against Russia or the United States if it comes to war. Even the much smaller military of Japan could defeat China just on the basis of superior military technology and war treaties.
The reasons for China’s military weakness are many, but much of it has to do with a lack of technological capability. China mainly relies on other countries to supply the necessary technology for weapons. Also, China still doesn’t outdo the United States in military spending.
There are deeper reasons, too. The communist party is attached to the military. Widespread corruption within the military, and within the government as a whole, actively stifles military function.