Build a wall?   Why?

This fortress built by Nature for her self
Against infection and the hand of war,
This happy breed of men, this little world,
This precious stone set in a silver sea
Which serves it in the office of a wall
Or as a moat defensive to a house,
Against the envy of less happier lands,
This blessed plot, this earth, this realm, this England,
This nurse, this teeming womb of royal kings,
Feared by their breed and famous for their birth,
Renownèd for their deeds as far from home
For Christian service and true chivalry