You can make a number of different arguments, but in my opinion, it was ultimately that he didn’t fully understand the amount of force that was going to come down on him to stop him. He did it all too fast. I think he should’ve fixed the country and then started his conquest once the dust had settled more. He should’ve known the whole first world was corrupted and would fight back, but his large scale invasions whipped them up into desperation, forcing them to use their whole power to stop him