There are many problems here. I mean, other countries have their own assassins, and there are things like the military and stuff. Surely everyone is going to react in some way, right?
Japan is economically significant, I doubt other countries would just let a terrorist organization take power and start a purge day one without any repercussions; if anything, it's the perfect time for a foreign power to try and seize Japan for itself under the guise of protecting it's people and the current government being too weak.
Well I will proceed not giving a fuck, people in this manga see their cars blowing up and people having sniper battles in broad daylight and they don't give a fuck, so I guess common sense here is wack and trying to make sense of things is just stupid of my part.