‘And knowing their thoughts he said to them, “Every kingdom divided against itself is brought to desolation, and every city or house divided against itself will not stand, and if Satan casts out Satan, he is divided against himself. How then will his kingdom stand?”
Jesus replies by showing up their false logic. Kingdoms where civil war is continually in progress always collapse. Households which are always at loggerheads do the same. Thus if Satan actually casts out Satan, he is in a similar state, and therefore his kingdom also will collapse. But everyone knew that Satan's kingdom was in fact to grow stronger towards the end, not weaker. How then could that be if it was subject to civil war? And besides, Satan had too much common sense for that. Thus it was quite clear that their assertions must be untrue.
Note the thought that Satan's kingdom is one kingdom made up of all the kingdoms of the world (see Matthew 4:8). The idea is that whole world lies in the evil one (1 John 5:19), apart from those in the Kingly Rule of God (compare Colossians 1:13).