The midnight raid, widely known as the "Boston Tea Party," was in protest of the British Parliament's Tea Act of 1773, a bill aimed at saving the foundering East India company by drastically lowering its tea tax and giving it a virtual monopoly on the American tea trade.
As if that weren't enough, the low tax even allowed the East India Company to undercut tea smuggled into America by Dutch traders, much to the consternation of the colonists.
When three tea ships arrived in Boston Harbor, the colonists demanded that the tea be returned to England. When Massachusetts governor Thomas Hutchinson refused to accede, Patriot leader Samuel Adams organized the "tea party" with roughly six members of his resistance group, the Sons of Liberty.
The British tea dumped in Boston Harbor that night was valued at approximately $18,000.
Outraged by this brazen destruction of British property, Parliament enacted the Coercive Acts (also known as the Intolerable Acts) in 1774, which required colonists to quarter British troops, established formal British military rule in Massachusetts, closed Boston to merchant shipping, and made British officials immune to criminal prosecution to America.
The colonists then proceeded to call the first Continental Congress, setting the stage for a Revolutionary War that would result in America winning it's freedom from the mightiest military force in the world.