What constructs a World War exactly?
History is the teacher. According to my own military education, there are generally three types, or levels, or military conflict, graded by intensity. However, it should be noted that within these scales are gradations.
1. Low Intensity Conflict - this type covers things like insurgencies, terrorism and guerilla campaigns that usually are not violent enough or frequent enough to threaten the overall political fabric, but could, over time, sap the resources of a nation and move to the next level. Often, they can be the result of or cause of societal fragmentation which sets the stage for increasing instability if not properly managed or defeated by the ruling powers of the state or region in which they are underway. Sometimes LIC's can multiply into the regional theater and threaten political, financial and social stability across a broad spectrum and bait greater national entities into getting involved. Negotiations and deals are made and broken on a nauseating basis, but these wars generally run in the background. These LIC's were common during the Cold War era and actually served as a sort of chess board for the Great Powers who wished to avoid larger conflicts or direct confrontation.
The danger of these is that if there are enough of them, LICs could set the stage for the next two types of conflict.
2. Mid Intensity Conflict - could class as a major war in relation to the former, but is generally contained within a region between two antagonists. Outside powers may be involved indirectly, and smaller LICs may stem from this type of conflict. They can also rank with sudden flash punitive campaigns of relatively short duration due to the types of forces employed - meaning major weapons systems or orders of battle like aircraft carriers or land armies spread over broad fronts. Still, major powers can get bogged down, and get stuck in attritional fights. Such types of warfare can also draw great powers of into broader more dangerous direct confrontation with each other, but often they will try to avoid the risk of going to the next level and contain the conflict, as in the former case.
3. High Intensity Conflict - Mid can turn to high, as in a coalition campaign, but this may not necessarily peak into a global conflict. The highest intensity, barring thermonuclear war, is called "Total War", where the entire resources of a nation are put into play to defeat their enemies. These types of wars may begin with maneuvers meant to crush the enemy swiftly, but they often tend towards attrition, where entire nations can be devastated and their resources sapped, their lands occupied and divided among the victors.
So, to answer "what constructs a world war", you have to consider the dynamics of complex alliances, old grudges and standoff that manifest in the first two levels of conflicts that finally boil over into a general conflagration. Studying what led up to the two World Wars is easy, but the precursors reveal much.
Since the introduction of nuclear weapons in 1945 and their mass proliferation by the so-called "superpowers" and some lesser powers, the chances of the supreme peak (that is, a potential extinction event as opposed to just a Total War) conflict has generally been deterred. The basic reason for this is "Mutually Assured Destruction", which implies that if A attacks B in a first strike, B's retaliatory strike will be launched immediately after it detects A's launch, and both sides will be ruined within hours. Any counterstrikes afterward will only add to "overkill" - there are no winners, only a dreary mass of burned and blind survivors with no effective government to speak of as they dwindle into extinction. This is what we fear.
I hope that clarifies at least that part of the OP. To put this in the context of current events is not easy. I would say we are in the buildup towards a potential global conflict that would probably find us fighting in planetary orbit, or even on the Moon, plus using militarized AI, robotics (drones), energy weapons and other innovations. And I say this because it would fit the pattern of technological evolution set by prior wars. But, as we see, the "old bloody infantry" in the attrition fight still plays a critical role. That our current convulsions would result in nuclear war, even a limited exchange,
is possible, as some have come to openly question whether or not such a fight could not be won. But miscalculations in warfare, as in politics, are inevitable. It doesn't mean that nuclear weapons would be used. In WW1, poison gas came to be used extensively with no decisive outcome, but it was hardly used in WW2, except against civilians.
One other note - because of the complex web of a shrinking world, it is entirely possible that "World War 3" could be fought in a very limited but drawn out way. The apocalyptic association of Total Nuclear War might be avoided, barring a madman in power or some mistake (as almost happened in 1983). Some authors I have read referred to the Cold War as this limited type of global conflict, using the Third World as a pun. But, basically, if you look to earlier times, like the 18th century, militaries might engage each other in duels and dances while governments still talked and even conducted trade. In this trend, there is just too much to lose, so the fight becomes almost ritualistic, though only future historians could make a broad assessment.
