As for what other battles might be as or more important; Gettysburg! If Lee is able to destroy the Army Of The Potomac, as planned, there is the possibility Lincoln does accept the Confederacy's bid for peace & the country is divided. The USA is plunged into a European style 100yrs War as North & South battle during the race west. Native Americans are likely wiped out. USA's participation in both World Wars is nonexistent. That may mean nothing in WW1 as the German's were on their way out by then, but it likely meant possibly another year of war & 100s of thousands more casualties. BUT, without at least USA's aide, it is highly unlikely Britain stands against Nazi Germany. Russia stands alone against all the Axis.