The Following Questions Refer to the Excerpt Below

The following questions refer to the excerpt below.
"ARTICLE XIII.
The tribe herein named,by their representatives,parties to this treaty,agree to make the reservation herein described their permanent home,and they will not as a tribe make any permanent settlement elsewhere,reserving the right to hunt on the lands adjoining the said reservation formerly called theirs,subject to the modifications named in this treaty and the others of the commander of the department in which said reservation may be for the time being;and it is further agreed and understood by the parties to this treaty,that if any Navajo Indian or Indians shall leave the reservation herein described to settle elsewhere,he or they shall forfeit all the rights,privileges,and annuities conferred by the terms of this treaty;and it is further agreed by the parties to this treaty,that they will do all they can to induce Indians now away from reservations set apart for the exclusive use and occupation of the Indians,leading a nomadic life,or engaged in war against the people of the United States,to abandon such a life and settle permanently in one of the territorial reservations set apart for the exclusive use and occupation of the Indians."
Navaho Treaty,Fort Sumner,New Mexico,June 1,1868
-Which of the following events led the U.S.into contact with the American Indians referenced in this document?

A) Growing numbers of immigrants to the United States
B) he completion of the Trans-Continental Railroad
C) The Confederate defeat in the Civil War
D) American victory in the Mexican-American War