Why did the north want to gain control of the Mississippi River

Nationalism led to major conflicts following world war i, especially when:

Plz ive been stuck on this question forever. in 1753 george washington was sent on a mission to a. build a fort at the point where the allegheny and monongahela rivers form the ohio river. b. attack the french at fort duquesne. c. attack the french at fort neccessity. d. deliver a message to the french demanding that they leave the ohio river.

Who was francis marion and how was he linked to the revolutionary war?