The Asian summer monsoon affects more than sixty percent of the world's population; understanding its controlling factors is becoming increasingly important due to the expanding human influence on the environment and climate and the need to adapt to global climate change. Various mechanisms have been suggested; however, an overarching paradigm delineating the dominant factors for its generation and strength remains debated. Here we use observation data and numerical experiments to demonstrates that the Asian summer monsoon systems are controlled mainly by thermal forcing whereas large-scale orographically mechanical forcing is not essential: the South Asian monsoon south of 20 degrees N by land-sea thermal contrast, its northern part by the thermal forcing of the Iranian Plateau, and the East Asian monsoon and the eastern part of the South Asian monsoon by the thermal forcing of the Tibetan Plateau.