Many will say that World War Three cannot happen, that nuclear weapons will prevent it. However, this assumes that World War Three has to be global thermonuclear war, rather than some repeat of the previous world wars.

Cities don't have to be leveled for nations to fight a world war. The US fought two world wars, and we never had our cities and infrastructure decimated. What I can imagine is a future world war where all the major players fight the war in the same way the US fought the two previous wars. Both sides contribute massive resources, adopt wartime economies, throw their whole populations behind the effort etc, but at no point do the various combatants directly attack the main territory and population centers of the other side. You could have a conflict where both sides lost millions of troops fighting it out in some third party territory, but the nukes never fly as all sides realize that invading the home territory of the others is suicide.