History painting, or historical painting, is a genre of painting that has two main aspects to it. First, the subject of history painting is historical (or Biblical) events. Second, history painting is generally meant to make a statement. A historical painting is meant to teach people certain lessons about history. This painting by West is clearly an example of this genre.
This painting was done by West in the early 1770s, almost one hundred years after the event that it depicts. It shows an event from 1683, in which William Penn concluded a treaty with a chief of the Lenape Indians. In this painting, West is trying to portray the American colonies, and the relationships between the colonists and the Native Americans, in a positive light. He is, whether intentionally or not, encouraging us to forget about the many conflicts that arose between the Native Americans and the European settlers. In this painting, there is no hint of avarice or duplicity on the part of the white men. Most of the whites stand back from the Indians; they are not trying to put any pressure on the Indians to give them land. The Indians appear to be considering and discussing the treaty being offered to them; there is no hint that they might not understand the real ramifications of the deal.
What we can see is that West is using painting as a tool to tell a certain story about America and its origins. He is portraying the colonists as honest people who made honest deals with the Native Americans. This glosses over the unequal nature of many of the treaties and the overall poor relations between the Europeans and the Native Americans that were much more typical of colonial America.