From a purely scientific point of view,
without the verdict of history, it can only be speculation. Even with the verdict of history, the outcome of conflicts is often viewed very differently by the opposing sides. Sometimes it is even viewed differently by commentators on the same side. I picked WWII as my cut off for precisely this reason.
Alternative history is a fun game to play, and it helps sharpen our reasoning and persuasive skills, but it can never prove anything. All historical events have unexpected and unforeseen consequences, not just wars. Trying to speculate about how world history would have changed if, say, the Confederacy had won the Civil War, is like trying to discuss the plays Shakespeare might have written if he had lived another ten years.