The question of whether Christians should have fought in the American Revolution comes down to a matter of personal convictions and the teachings of individual Christian denominations. Some denominations take a strictly pacifist approach, declaring that violence is never justified for any reason. Others say that armed conflict is scripturally allowed in a righteous cause.
In the New Testament of the Bible, in Matthew, chapter 5, Jesus admonishes his followers early in his ministry to "resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also" and to "love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you and persecute you." Later, however, in chapter 22 of Luke, when he is about to be captured by the Romans, tried, and put to death, he tells his disciples, "He that hath no sword, let him sell his garment, and buy one." His disciples answer that they have two swords, and he replies, "It is enough." He was obviously not against the carrying of weapons under certain circumstances.
Another pertinent passage is the beginning of chapter 13 of Paul's Epistle to the Romans. Paul admonishes that Christians should be "subject unto the higher powers," meaning governmental authority. However, some Christians insist that this is a warning against anarchy, and it does not preclude overthrowing an oppressive, tyrannical government. In the eyes of the American colonists, King George III had broken British law as well as scriptural tenets in his harsh treatment and violent oppression, so they were no longer bound to be subject to him.
In fact, many of the colonists in the New World felt that their struggle for freedom was a righteous one and therefore justified in the eyes of God. It was significant for them that they were not the aggressors; they only rebelled after years of violent military subjugation by the British. The opinion of the colonists' own ministers gives an indication of the Christian response to the American Revolution. By and large, Christian ministers approved of the Revolution. They felt that it was the Christian duty to resist tyranny. They preached the righteousness of the American Revolution from their pulpits, served as chaplains with the militia, and even took active part in the fighting. In their view, the struggle was fully justified by scripture because they were defending their homeland and protecting the innocent.
With these ideas in mind, you can form your own opinion about whether Christians should have fought in the American Revolution.