On 25 November 1970, Mishima and his 4 followers who were part of his military LARPing troupe "Tatnenokai", interrupted the lunchtime of JSDF outpost by triggering a hostage crisis and stabbed a few while shouting incoherent shit about overthrowing the Constitution.
After only slogging through 7 minutes of his 30 min prepared speech, partly because overhead helicopters drowned out whatever he said and partly because increasingly hangry JSDF men kept booing him, he attempted seppuku but sucked at it so he kind of lay flailing there. The original intention, apparently, was to use his blood to write his Will.
One of his followers, Masakatsu Morita, was supposed to behead him but he mis-struck twice because after that he was also supposed to commit seppuku and I guess seeing Mishima flop around like that weakened his resolve. Hiroyasu Kouga ended up beheading Mishima, and did 4 years for assisting a suicide.
After this, the JSDF inserted a pledge clause including "to uphold the Constitution and the laws of Japan".
Today Mishima is loved by Japanese rightoids who try to scrub the internet of how retarded his final day on earth was, except you can totally see photos of his beheaded corpse lol.