It was a biblical act of self-defense against a foreign invader. That foreign invader, the nation of Great Britain, acted without legal basis for its attempted conquest of the people of America. The Bible teaches that the Christian owes due allegiance and submission to the government authorities in most, but not all cases. Normally, we are to obey every ordinance of man. Exceptions include those cases where the State commands the Christian to do what Christ has forbidden, or prevents a Christian from doing what Christ has required.
All authority is limited including government authority.
No monarch or civil leader has absolute power, nor does he have the right to expect others to reject the law of God. Peter gave Christians a very practical example of State tyranny and of the biblical necessity of civil disobedience when he refused to honor the unjust prohibitions against preaching. Concerning self-defense, the Scripture makes it clear that men are permitted to use lethal force if necessary when acting in defense of self or others, be the threat personal or national. Finally, while Christians may be forced to temporarily submit to others who threaten them with violence, the mere fact that a person claims authority and attempts to back up the claim with force does not validate the individual as a lawful authority. This is a fundamental difference between Rome at the time of Christ, and Great Britain at the time of the War.