I just saw this question, and I guess I should probably try and answer it, as late as it is. I'm going to assume that you're referencing Chinese armies in comparison to the West, as that's really the only valid comparison.

A bit of backstory - at the beginning of the 18th century, China under the Manchu Qing rule was at the peak of its power and prosperity, as the Kangxi emperor and the following rule of the Yongzheng and Qianlong emperors essentially consolidated China into the China you see today. Trade with the West was confined to certain cities as a response to the expanding trading routes established by European empires, essentially giving Chinese merchants a significant influence and monopoly on price.

The British in particular wanted Chinese porcelain (china), tea, and silk, and in trade the only thing that was in super high demand was opium - in response to the growing addiction of the Chinese populace to opium, the central government ordered an end to the opium trade, leading to the beginning of the First Opium War (1839 - 1842).

This war was short due to the significantly better equipped British navy - they had advanced gunpowder weaponry, modern rifles and heavy artillery, as well as superior ships in comparison to the archaic Chinese navy - the supreme reign of British ships over the sea surrounding China in turn crippled the financial situation in Beijing, causing the Chinese surrender and the signing of the Treaty of Nanjing.

This forced them to give up several ports along the Chinese coast, as well as giving the island of Hong Kong to the British empire where it remained as a colony for more than a century. Similarly, rebellions within China following the unequal treaties eventually marked the turning point of the dynasty where it fell into decline. As a continuation of this into the early 20th century, China was comparably weak in the Asian region and never really regained it's power until the late 20th century.