The boundary of a kingdom is not fixed by geography, but is determined by the authority of the king.
A nobleman who lives within a kingdom may refuse to accept the authority of the king. If the king cannot control the nobleman, the people controlled by him are not part of the kingdom. Outlaws do not accept the authority of the king. Although they live within the geographic boundaries of the kingdom, they are not part of it. They do not acknowledge the laws of the king, so they do not receive his protection.
Geography is only relevant, if it constrains the expansion of authority. Extending human authority over mountains and across seas is difficult, so most human kingdoms are limited to a particular region. The kingdom of Israel was a geographic kingdom. The king’s authority waxed and waned as its geographic boundaries expanded and contracted.
The scope of the Kingdom of God is not limited by geographic barriers. The boundary of the Kingdom of God is defined by attitudes to his authority. It extends wherever God’s authority is accepted and his decisions are obeyed. The Holy Spirit can carry the authority of God throughout the world, so his Kingdom can extend into all the earth.Kingdom Authority, p.9