Were the American colonists justified in waging war and breaking away from Britian?
The American colonists were justified in doing this simply because their colonies had become too big and too important to be treated as a colony by the British. The British should have given the colonies some autonomy, but they did not. The analogy I like to use is that of teens and their parents. Parents have to give teens more independence as they grow up. If they do not, the teens may justifiably rebel.
The British were not, on the whole, brutal or oppressive towards the colonists. However, they would not let the colonists have much in the way of self-rule. This had been fine when the colonies were still small and economically weak. By the 1760s and 1770s, however, the colonies were "teenagers." They were big and strong enough to expect some autonomy. When Britain reacted to requests for autonomy by being more strict, the colonists were justified in rebelling.
This question can be looked at from both sides. Many American colonists felt as though they were justified in waging war and breaking away from Britain. Jefferson, who wrote the Declaration of Independence, and other figures active in the American Revolution argued that John Locke's social contract, which stated that the monarch must follow the will of the people, justified the American break with its mother country. Britain was, many colonists felt, no longer following what the colonists wanted and was taxing them without giving them fair representation in British Parliament.
Britain, on the other hand, justified its reinforcement of the Navigation Acts and the taxes it levied on the colonies by arguing that these taxes were needed after Britain had defended the colonies during the Seven Years' War (also known as the French and Indian War, which lasted from 1756 to 1763). The British responded to American demands for direct representation in the British Parliament by saying that no one in Great Britain was represented that way in Parliament. While some people in the colonies wanted to stay tied to Great Britain, the Patriots won out after the Battles of Lexington and Concord in 1775.