Final answer:
The event being described is the Proclamation of 1763, which forbade colonists from settling west of the Appalachian Mountains.
Step-by-step explanation:
The event being described in your question is the Proclamation of 1763. This proclamation was issued by the British Crown and it forbade colonists from settling west of a line drawn along the Appalachian Mountains.
The main reasons for this proclamation were to prevent conflicts with Native Americans and to maintain control over the colonies.