In The Singapore Story, Lee Kuan Yew pointedly explains that the
three and a half years of Japanese occupation were the most important of my life. They gave me vivid insights into the behaviour of human beings and human societies, their motivations and impulses. My appreciation of governments, my understanding of power as the vehicle for revolutionary change, would not have been gained without this experience.
First of all, he observed "a whole social system" built upon assumptions of British military and cultural superiority "crumble suddenly before an occupying army that was absolutely merciless. The Japanese . . . were hated by almost everyone but everyone knew their power to do harm and so everyone adjusted." People who accepted the Japanese as their new masters prospered, whereas those who did not lost money and status.
Second, he learned that fear of brutal punishment can deter crime:
The Japanese Military Administration governed by spreading fear. It put up no pretence of civilised behaviour. Punishment was so severe that crime was very rare. In the midst of deprivation after the second half of 1944, when the people half-starved, it was amazing how low the crime rate remained. . . . As a result I have never believed those who advocate a soft approach to crime and punishment, claiming that punishment does not reduce crime. That was not my experience in Singapore before the war, during the Japanese occupation or subsequently. 
"I learnt more from the three and a half years of Japanese occupation," he states again a few pages later, "than any university could have taught me" (77). A third lesson, in particular, remmained with him his entire life -- that power can make people change their ways of thinking and acting:
I had not yet read Mao's dictum that "power grows out of the barrel of a gun", but I knew that Japanese brutality, Japanese guns, Japanese bayonets and swords, and Japanese terror and torture . . . could make people change their behaviour, even their loyalties. The Japanese not only demanded and got their obedience; they forced them to adjust to a long-term prospect of Japanese rule, so that they had their children educated to fit the new system, its language, its habits and its values, in order to be useful and make a living. 
Lee Kuan Yew. The Singapore Story: Memoirs of Lee Kuan Yew. Singapore: Times, 1998.