Living In Fear

I vote water #1. Oil has alternatives, a huge portion of our demand for water comes from creating food, desalination or altering the supply of fresh water in any meaningful way is incredibly intensive, and the fuse from 'supply is interrupted' to' societal collapse' is very, very short.

c0y0t3_sly

War Hawking

I would venture it'll be a dispute over oil or another vital natural resource. Nobody wants war as it hurts the domestic politics at home in the long run, especially in a federal or parliamentary system. But if the domestic situation is so bad (stupid high car fuel, heating, energy, etc) a major hegemon may choose war to either distract the public from the problem in the short-term or attempt to solve it outright, by invading and annexing another, probably less powerful state/area who already has built in access to that resource. (Persian Gulf, Arctic, East China Sea).

Can imagine it would snowball from there, especially if and when other hegemon's feel their own supply lines are now in jeopardy.

EricLaGesse4788

Literally On The Brink

The EU has a financial crisis, sparking a rash of Brexit-like movements across the continent. Desperate to hold things together, the EU, backed by Germany, will declare that no one is allowed to secede from the union without their permission. France will try anyway. Germany invades France again.

The English speaking world might give France a hard time, but goddammit, France is like a younger sibling to them. Only they are allowed to pick on France, so they show up for round three with Germany.

With the US and the UK tied up in western Europe, Russia and China will see their chance and try and take over the former Soviet bloc states and southeast Asia respectively. The parts of Africa and South America that aren't exactly stable even in the best of times break down completely while the rest of the world is too busy fighting each other to keep an eye on them, and the chaos slips over the borders and spreads throughout the whole region causing diplomatic relations to break down and everyone starts pointing fingers at everyone else. Eventually someone points a gun instead of a finger and all hell breaks loose.

righthandoftyr

This All Could Happen Shortly

There are lots of possibilities, but the most likely are:

- North Korea launches an attack on Japan or South Korea

- Iran versus Saudi Arabia versus Israel leading to an all-out war in the Middle East, from Turkey to Indonesia.

- A successful cyber attack by North Korea, Russia or China against the USA, then a counter-attack by the USA, leading to widespread chaos and a global economic and infrastructure crash. This would be an entirely new situation and lead to martial law and massive military controls in many countries. Democracy is discarded by military dictatorships and new border wars flair up globally. A very real and very ugly scenario.

Count2Zero

At This Point, It's Willful

Ignorance. In a world where information is at our fingertips, people have grown dumber than ever. Nothing is questioned anymore. People make their own little bubble of reality and are afraid to venture beyond.

Jehoel_DK

The Thing That Causes Every Other War

Misunderstanding, mirroring, a remarkably improbable mistake all in an environment where all the rulers think they got stuff under control. Most big things are really just a series of small things that happen one after another is a way that seems obvious and predictable only in hindsight.

Nemacolin

Germany Part 2

China is the only nation with the size and (soon to be) power to enter into a conflict with the kind of massive, existential stakes necessary to drag the great powers of the entire world into a war.

It would take something like an expansionist push to actually conquer large swaths of territory including that held or considered critical to the West. However, there are numerous places that extremely destructive wars could break out between individual powers, and with the tendency for those wars or civil wars to internationalize...who knows.

blyat55

-Cough Cough- Trump

Someone is going to either Tweet, Snap, or Facebook something overtly offensive and it will just ignite a chain of events that we will never get back.

geekygamergirl2081

The End Of The World

I reckon that the cause of World War 3 would likely be a nuclear war and World War 4 would just be a Nuclear Winter and the consequences of the War, where us, humans would fight to get basic necessities to survive,ie food and water.

I do not know with what weapons World War III will be fought, but World War IV will be fought with sticks and stones —Einstein

bad_karma118

The Profit Of Disaster

I'll give a serious answer. Climate Change triggering wars. How?

1. Places like North Pole (or close to it) were inhabitable forever. Due to climate change, ice is receding from these areas and we have found a lot of resources like natural gas in this area. This has created a lot of disputes between Russia and US/Canada. Every year, the chance of a flair up between these powers continues to rise.
2. A lot of countries will lose significant land or in worse cases submerge completely under water. Example: Bangladesh. This can cause large refugee crisis and could easily trigger wars of territorial conquest

SlytherinSlayer