Mandarin Chinese as we know it today is actually Manchurian Pekinese as standardized by Qing Emperor Kangxi when he adopted the policy of Man-Han one family one country. It was the second biggest standardization of Chinese all across China after Emperor Qin Shihuang standardized traditional Chinese characters. Ironically, both Qin and Qing were not credited for that but Han took the credit all the way. Mao Zedong after CCP victory over ROC standardized simplified characters because as wishing to speed up the learning process for illiterate kids and as a calligrapher himself, ordered the standardization. It's nothing new, in existence for centuries, mostly used by calligraphers as short hand calligraphy. The only difference made then was that the whole set was standardized. People like Mao Zedong, Deng Xiaoping all the way to as recent as Wen Jiabao and Hu Jintao never spoke in the standard Chinese dictionary prounciation and tones.
Japan after rising from the ashes of WW2 defeat also recalibered their mixed usage of Japanese kana and Chinese characters independently. That resulted in some Chinese characters and Japanese kanji being written a bit differently, simplified in a different style, missing or adding a stroke or dot.