The Opium Wars can be called a turning point in world history because of their impact on China. With China having become an important country, its history relevant to world history.
The Opium Wars helped to do two things. First, they helped to undermine the Chinese government. They showed how week and ineffectual the government was. This helped lead to the fall of the Qing Dynasty in the early 1900s. This then led to the Chinese Civil War and the eventual triumph of communism in China.
Second, the Opium Wars have helped to shape Chinese attitudes today. The unequal treaties imposed on China helped to make China somewhat resentful of outside forces and somewhat nationalistic. This affects Chinese attitudes towards the West today.