I don't think there is one single war that has single-handedly defined the present day as all wars have had a significant impact on how things were. Like very small wars have effects on bigger wars that follow. So, technically unimportant or atleast unknown wars could have great effects on thew wars that are commonly attributed to be future altering. All wars/conflicts/battles have significant effects on the way things are now. The one with the biggest impact however, in my opinion, would have to be the Gaulic war fought by Julius Caesar. Had he not put a decade of his hard work, blood, sweat and the lives of Roman Soldiers into that war he wouldn't have returned to Rome to conquer the republic and establish his dictatorship, ushering in the era that so defined the modern world that had it not happened the world would be incredibly different and would have been sent on a different route. I'm not saying it wouldn't be advanced, nor am I saying it would be more advanced. I truly can't tell. But it would definitely had the most profound impact. There are also many other reasons why I attribute it to be the most important, but I don't want to delve to far into them at the moment.