"M" is for Manifest Destiny
The secret letter is
MThis round will concern itself with events that led towards Westward Expansion, from the years 1801 to 1900, on the North American continent but mostly the United States. The subject is (loosely) "Manifest Destiny", excluding the Civil War period which has been covered in part. Again, M can be a first or last name, but not an article or common object, unless the answer specifically is a title, like "The Battle of Moose jaw"..
Category 1: A Native American Military Leader, active between 1801-1900, engaged in hostilities towards the United States.
Category 2: A battle in the Mexican American War.
Category 3: A Native American tribal name (the tribe, not an individual)
Category 4: The name of a famous Railroad "Robber Baron"
Category 5: A Famous Woman Pioneer, living somewhere in this time period
Category 6: The name of a town that a "Rush" took place in.. gold or silver, makes no matter.
Good luck, history fans!