Historians who try to explain Union victory and Confederate defeat during the Civil War approach that question by asking several questions (or at least implicitly offering their answers). The first question is whether Union victory and Confederate defeat were, in fact, inevitable. Was there any way for the Confederacy to win, or was it a lost cause from the beginning?
How one answers this question shapes how one approaches the study of the Civil War … or at least it should. However one answers that question determines the next questions one asks and which issues one explores. Say you think the outcome of the war was inevitable (which is different from determining when the outcome of the war became inevitable, because that assumes that at some point it was not inevitable). If you believe that, then the study of why certain battles turned out the way they did becomes less important, because the ultimate outcome was set, anyway. It doesn’t matter whether McClellan was slow or Longstreet lost Gettysburg or Thomas was undervalued, because, whatever one may find interesting about those queries on their merits, they don’t shape the ultimate victory, because you’ve already deemed it inevitable. Thus, there’s no need for turning points or contingency. One looks instead to underlying factors such as resources to explain why it was inevitable: the issue of popular will becomes irrelevant.
However, if you do not believe the outcome of the war was inevitable, then you have to deal with actors, decisions, and events. Oh, you can talk about resources, for example, but you’ve acknowledged that the outcome was not preordained, and in fact you have to think in terms of counterfactuals as well as contingency and turning points. If certain events led to Appomattox, then different outcomes would have led to different results, and then you have to identify what made the difference. For example, I happen to think it is harder to imagine Union victory without Grant. Take Grant out, and you have to presuppose that someone else would have come along with a mix of talents and skills approximating his. Take Grant out, I’ll add, and it becomes more difficult to consider the rise of Sherman, who found in Grant his North Star.
I don’t happen to think that Union victory was inevitable. Nor do I think that the events in the East in 1863 offered a so-called turning point. If 1863 was in any way a turning point, it was because the victories at Vicksburg and Chattanooga established Grant as the coming man, and that led to the two turning points of 1864, Grant’s rise to top command and the success of his grand strategy in September 1864. The Union may have had superior resources, but Grant was the first commander to use them in coordinated fashion and to make that superiority tell.
As to how the Confederacy might have won, I’ll save that for another time.
Others may differ, of course, and that’s why we have a comments section.