What caused the American colonists' break with Great Britain?

Attempts by the British Parliament, with the support of the King, to exercise legislative powers over the colonies, particularly the power to tax, which had not been effectively exercised previously in colonial history. The colonists were used to making their own laws, and setting their own taxes, through their own elected legislatures. Since they had no representation in Parliament, Parliament's claim to be able to legislate and tax then itself, and enforce those laws with officers answerable to it rather than the colonies, meant the colonists would have no effective means of exercising and protecting their rights. Parliament could undo any of their laws and and tax them into utter poverty on a whim. When Parliament would not acknowledge the justice of the colonists grievances, and it became apparent that the king completely supported the parliament, the colonists decided to declare independence. For a more detailed accounting of just what the colonists were concerned about, read the Declaration of Independence, not just the first few lines that everybody memorizes in school, but the whole thing. Jefferson gives a long list of charges against the king and Parliament showing why the king was a tyrant unfit to rule.