The war in heaven is sparked when the rebel angels following Satan decide to try and overthrow the "tyrant" God. This is mirrored in the political upheaval of seventeenth century England after the overthrow and execution of it's own tyrant, King Charles I. Executed after a bitter struggle for power with the Parliament of England, Charles I was accused of tyranny for manipulating the monarchy in order to further his own, personal agenda. Milton wrote on the side of Parliament, and in his polemical tract, On the Tenure of Kings and Magistrates, argued for the natural right of the people to expel and execute tyrannical kings.
This is reflected in Paradise Lost through the rebellion. Like Milton, Satan fights against a "tyrant" and argues for freedom. However, Milton makes the distinction, through the pious Abdiel, that God is not an earthly king, and cannot be treated as such. To Milton, God isn't a tyrant-king, and Satan isn't a defender of liberty; instead, the fallen angel is only a perverted manifestion of the poet -- fighting on the wrong side of reason.