Why has there not been another World War...?

I honestly believe when the countries are sick of fighting the proxy wars they currently are they will actually confront each other head on. This most likely war will be between China and the USA. Countries like Europe, canada and Russia will ahave to decide which side to be on as both are massive players in the World. The South China sea looks like the most likely place to start as there already has been clashes with Chinese and American ships. When China dumps the usa dollar that will definitely start the war as this would destroy America's currency over night.