jiashah is right that it will be the water for which war starts,and she is right that it may be starts from this region and main characters can be Pakistan and India.
Because of the climate change and growing human population now its tough to manage a very little water which is useful for life and agriculture,
only two or three percent of total water on earth is useful for the life and agriculture ,and because of growing population,demand is increasing which is
alarming about that danger which will be occur in our future,and by the time when things become matter of survival then probably such issues will be
on the priority of countries and
may be the 3rd world war will be for the surveillance of the nations and countries.
And if we think that it will starts from Pakistan and India then
then there are some reasons of it,
we can see water crises in our country, even our provinces are fighting for the water.
India also planing for the controversial "KISHAN GANGA" Dam on Indus river which results shortage of water in our country,
means cost of that water will be payed by us.
our economy relies on our agriculture and if India keeps completing such projects then most probably it will makes trouble for us ,and if it becomes matter of survival of us then definitely our decisions will be tough and that result can be in shape of a war.
But there are international court which resolve water disputes and Pakistan and India can resolve it but there is little chances of it because in history India blocks water of Pakistan.
And you see when water will be the core issue of countries then oil or other form of energies become the second priority,and most probably 3rd world

war will be not for land , oil , gas or gold.

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Life is what happens to you while you’re busy making other plans. –John Lennon