There were many reasons why Batman did not prevent the departure of Gotham from Sky Island, but the safety of the Batman Family was the biggest reason.
The war between Darkseid and the Green Lantern Corps was bound to be lengthy and the outcome seismically challenging, but Batman knew that Darkseid would not leave him alone. He had always been interested in the half of the equation Batman held in his hand.
Considering Earth is located remotely and relatively weak, there was no need to dispatch a fleet. The best method would still be to send an assassin to kidnap or kill. If he couldn't deal with Batman, Gotham Sky Island floating over Earth's head would be the perfect target.
Haven't Darkseid sent people before? Although they did not successfully kidnap anyone, it has given Batman a wake-up call. This great sovereign was not a gentleman of integrity. Home invasions and kidnappings were his specialty, so he might as well move the house away entirely.
After Gotham moved to Hell, Batman felt reassured, but Hell was still not far enough. People like Constantine could come and go freely. Just like the time Robin was transferred, it would be a good idea to take Gotham to a farther universe for refuge.
Therefore, he allowed Gotham to go to an unfamiliar universe. Still, he did not anticipate that this universe was experiencing dramatic changes, and Gotham's participation would, to a certain extent, change the situation.
It's not surprising that the lunatics in Arkham Asylum would go to New City, but it's strange that they all fell silent after rushing into New City.
Batman quickly realized that they could have replaced the counterparts, the most obvious of which was the Penguin Man.
Penguin Man was the mayor, with more opportunities to appear in public. Some subtle anomalies in his movements were clearly visible in the media coverage, and Batman almost instantly determined that the Penguin Man had done a cat swap.
The Joker came next. Batman had never seen the Joker in New City, but he knew his universe's Joker too well. He also appeared on the news media, and the details on the front-page photos confirmed to Batman that this was the Joker he had just locked in Arkham Asylum.
Then there was Bruce. Bruce's arrival marked a turning point. Batman couldn't spare the time, but he hoped to stabilize the situation in Sky Island. So he thought of Professor Shearer's student, who was also a Batman. He was the only Batman that he could contact at the moment.
So Batman summoned Bruce, and from the photos related to Bruce Wayne shot by the media, Batman could see that although the local Batman and Bruce looked very alike, later Bruce still replaced the local Batman.
With the three main characters of the key turning point assembled, Batman now needs to investigate what exactly the Andrewkin incident that led to the outbreak of the war was all about.
On the afternoon of the 26th, the Joker appeared on the screen, claiming that a reporter had been kidnapped and revealing his relationship with Batman. Bruce Wayne went to the rescue, the rescue failed, and Andrewkin fell to his death.
On the same evening, 283 media outlets in New York and Gotham republished the report. There were more than 60 influential newspapers, and over 40 news media pointed their fingers at Bruce.
On the morning of the 27th, all the front-page newspapers were reporting on this incident. A lot of fake news and unfounded speculations appeared on the pages, and the focus of most media reports was on Bruce rather than the culprit, the Joker.
Up to this point, Batman can confirm two things. At this point in time, Bruce was still local Bruce, but the Joker had become the Joker from his universe.
It was clear that local Bruce could not handle this situation. He did not take any useful measures after the incident occurred, neither managing to reverse public opinion nor catch the Joker.
On the morning of the 28th, another bombshell news appeared, Mayor Oswald Cobblepot attempted to kill Bruce Wayne, and Bruce Wayne fell, was taken to the hospital for emergency treatment, and has since disappeared.
That afternoon, Mayor Oswald Cobblepot was arrested but not taken into custody, becoming a fugitive, and was lost for quite some time.
On the morning of the 31st, the parade set off from Brooklyn, New York, circled half-way around Brooklyn, crossed the Queens District, and entered the upper reaches of the Gotham River, and the war fully erupted.
Then the question becomes apparent — why would Penguin Man want to kill Bruce Wayne?
There's no reason for it. Oswald Cobblepot didn't have such motive or courage. The problem must undoubtedly lie with Bruce Wayne himself.
Batman arrived at the mayor's office, which has been sealed since the shooting occurred. Under multiple pressures, no police were able to investigate the case. The charge for arresting Penguin Man was not attempted murder. There was no need for evidence examination, so no one had touched the crime scene.
The blood-soaked sofa, barely noticeable footprints, traces of firearms left on the coffee table...
Numerous clues passed through Batman's eyes to his brain, and almost instantly, Batman determined that Bruce shot himself.
Penguin Man left marks on the sofa, not just because he was fat but because he would sweat more than the average person due to his size, so he used a more potent antiperspirant. The fragrance would leave traces on the fabric sofa for more than half a month and Batman could easily determine his sitting position.
The position where Bruce sat was difficult to determine, but based on the direction of the footprints and Batman's behavioural pattern, he likely sat in front of the Penguin Man.
They both sat facing each other, on opposite sides. Penguin Man's dominant hand was his right hand. If he fired the gun, it should hit Bruce in the left chest. Even if he deliberately aimed at the right chest, the bullet should have entered vertically, not at an angle as it was now.
Clearly, Bruce held the gun with his left hand and fired a shot at his own right chest.
From the scratches left on the table by the firearm, it was clear that the gun had moved from Penguin Man's hand, sliding across the coffee table to Bruce. This suggested that Penguin Man had influenced Bruce in some way, causing him to shoot himself.
But Batman immediately dismissed this hypothesis. Penguin Man could not manipulate Bruce, for Bruce's mentor was Schiller Rodriguez.
Moreover, having Bruce collapse in his own office would not benefit Penguin Man at all. Batman knew very well that the populist media hailing Penguin Man as a hero would not be any advantage for him.
You do not bite the hand that feeds you. Penguin Man had made his way into the elite circle through bribery. Offending these individuals, who were essentially his bread and butter, for some measure of fame, was a step any sensible person would avoid.
So, it could be Bruce's scheme, aimed at completely blocking Penguin Man's ascent.
But what followed was unexpected. Penguin Man was arrested, handcuffed, but ultimately not captured. He even managed to run into the slums and rally a protest group to send to Gotham.
After studying the events, Batman came to the conclusion that Penguin Man had been manipulated from the very beginning. Penguin Man had rallied the group out of self-preservation; he organized the parade to restore his reputation, and these actions aligned with his behavioral pattern.
Batman reviewed the hospital's surveillance and confirmed that Bruce had indeed been seriously hurt and had been on life support for more than 20 hours following the incident. He still remained unconscious.
The medical records were detailed and complete, and it seemed that Bruce had risked his own life to potentially bring down Penguin Man.
Framing Penguin Man was indeed a cunning move, so Batman could not ensure that this wasn't part of Bruce's plan.
Consequently, after all of the twists and turns, everything returned to the starting point. If Joker didn't kidnap Andrewkin, Bruce wouldn't have lost his reputation. Thus, Penguin Man wouldn't have become a hero for allegedly shooting Bruce, wouldn't have been able to rally a team, this team wouldn't have entered Gotham, and a fiery battle wouldn't have erupted.
Joker is indeed the root cause of the issues.
As predicted by the Joker, after much investigation, Batman discovered that the spark that ignited the conflict was the Andrewkin case. Joker was the puppet master behind the Andrewkin case, and he ended up destroying New City.
Batman trusted his investigative skills and judgement more than Joker's deceptive words. He didn't care for Joker's cryptic parables, so he confronted Joker once again.
"I told you, Batman." Joker lowered his head solemnly while sitting on the bed. As he spoke, his lips kept squirming, showing glimpses of his teeth.
"You really disappoint me." Joker tilted his head stiffly like a zombie and continued, "You don't understand me at all. Just like you don't understand yourself."
"Do you want to change your career and become the Riddler?" Batman asked sarcastically.
Joker remained silent. No matter what Batman did, he could not make Joker speak. It was then that Batman realized that there might indeed be some issues with his hypothesis.
What led Batman to this conclusion was that the Joker refused to admit his guilt.
Whenever Joker had previously committed a major crime, he would eagerly seek Batman's reaction and opinion about his behaviours, constantly urging Batman to laugh at his actions.
But now, despite causing such a big mess, Joker was denying his involvement like a petty thief trying to dodge blame in front of the police. This was unlike Joker
Joker was the madman Batman understood best. Batman had witnessed firsthand many of the messes Joker had made - bombings, kidnappings, and countless bizarre and terrifying homicides.
But these crimes were not the final goal, but the process. Joker didn't commit crimes to bomb people or kidnap for ransom. His crimes were committed in pursuit of his artistic aspirations.
As seen in the Andrewkin case, Joker's motive for kidnapping and forcing Bruce to let go of Andrewkin, resulting in his death, wasn't essentially to kill the victim, but rather to overthrow Bruce's psychological defenses.
Joker's art was to see people in pain and collapse, to see them struggling in the quagmire of human nature.
War was certainly painful, and many people would die, but their deaths were too trivial and devoid of any beauty. The process and result of these people's deaths could not provide any material for discussing humanity and artistic philosophy. Joker saw this as an unforgivable waste.
The Andrewkin case was interrupted by Bruce's startling shot, and Joker wouldn't let it go. If the chaos outside really was his doing, he would be busily rehearsing right now, and the moment Batman arrived, he'd present him with a huge surprise.
But he didn't. He gave up struggling and let Batman send him to Arkham Asylum.
Batman also figured out why Joker, a hungry ghost who was denied food, preferred to sit idly in the Arkham Asylum rather than participate in the commotion outside—no matter what, he wouldn't bear the sight of capitalists pouring milk into the river.