 Early in July, the fall of those fortresses opened the entire Mississippi to Union shipping and freed "Sachem" for duty in the Berwick Bay–Atchafalaya area which occupied the gunboat during the summer.Early in September, "Sachem" was assigned to a joint Army-Navy expeditionary force mounted at New Orleans to attack Sabine Pass, Texas.Possession of this port would close another important Confederate blockade running center and provide the Union with a base for a thrust into the interior of Texas.