Shortly after Chinese troops stormed into Tiananmen Square in June, 1989, the then-mayor of Beijing gave a lengthy report that, for 23 years, has formed the bedrock of the Communist Party’s justification for the use of lethal force against unarmed protesters. Now 81, battling cancer and fighting to salvage his reputation after a corruption conviction, Chen Xitong has declared in a new book that the bloodshed was “of course a tragedy that could have been avoided and should have been avoided.’’.