The Prohibitory Act of 1775 was passed as a measure of retaliation by Great Britain against the general rebellion then going on in the American colonies, which became known as the American Revolutionary War (or, in the UK, the American War of Independence).
The Prohibitory Act served as an effective declaration of war by Great Britain; a blockade being an act of war under the law of nations. The colonies and Congress immediately reacted by issuing letters of marque that authorized individual American ship owners to seize British ships in a practice known as privateer; further, the act moved the American colonists more towards the option of complete independence, as the King was now declaring his "subjects" out of his protection, and levying war against them without regards to distinction as to their ultimate loyalty or their petitions for the redress of grievances.
With the contemporaneous importation by the British of bands of foreign auxiliaries into the American colonies to suppress the rebellion by sack, pillage, fire, and the sword (the infamous Hessian), and the stirring up of hostile bands of Native Americans on the frontier by the King's men to raid the colonists, it became clear, even "self-evident" to the colonists that they would neither find liberty nor security under the King's protection, and thus, they exercised certain inalienable rights, and a rebellion turned into a war of national independence.
