What caused the united states to enter into a full-scale war in vietnam?

What do historians call the political,social,or cultural circumstances surrounding an event?

List three reasons why explorers began to explore new, and old areas. [[ps: doesn't explicitly say where. i think it's more in general.

"the plan of removing the aboriginal people who yet remain within the settled portions of the united states . . approaches its consummation. . an extensive region . . has been assigned for their permanent residence. it has been divided into districts and allotted among them. many have already removed and others are preparing to go. . " removal of southern indians to indian territory, 1835 this passage is most closely related to a) imperialism. b) the civil war. c) westward expansion. d) the french indian war.