In early 2016, US congress passed a bill that allowed relatives of the victimes of the 9/11 attacks to sue Saudi-Arabia. This, of course, put a strain on US-Saudi relationships, but it was nothing that a couple oil deals couldn't smooth over. It did, however, open the door to a whole new category of lawsuits.
Since Saudi-Arabia could be held, at least in part, responsible for the 9/11 attacks, this meant that other countries, too, could be held responsible for terrorist acts that were committed by its citizens, or planned on its soil. And it wasn't long before french citizens launched a lawsuit against Belgium.
The belgians thought that they, too, should be awarded some form of compensation, and sued their own government. After that, all hell broke loose.
Governments started suing each other, people sued the families of both those responsible for attacks, and those present at the attack but who got away unhurt, because they could have taken the place of their loved ones, and might have avoided doing so by "treachery or deceit", as was written in one filing.
Of course, not wanting to get left behind, Daesh, Al-Quaeda and others started suing too, mainly on the ground that lax security had permitted their members to become criminals and live in infamy, and also get sued a lot, something for which they wanted compensation from the governments responsible. It did not take long for wars to turn into legal battles. And once the lawyers took over, things really started to get ugly.