One thing we can be absolutely sure of is that if a war breaks out between China and the USA, or the USA and Russia, or the EU/NATO and Russia, it won't cost many civilian lives, and the Earth will also survive. However, I firmly believe that Europe and the USA will be technologically set back to the Stone Age, and above all, I don't think they will be able to withstand the barrage of cyber attacks they will face.
I don't have the imagination to envision the scenario, but it will have fatal consequences worldwide without directly costing human lives.