I think this century will see this giant war.
There're three remarkable reasons:
1. Natural reason.
2. UN failure
3. De-Balance of Powers.

Natural Reason:
Its very quite clear that population of the world is increasing day by day, and now it is touching 7th billions figure.
At other way the human resources are same as these were at earlier, i.e. Water, food, land, etc.
As Malithus Theory, Its clear that nature itself makes balance between population and resources, by wars, disasters, diseases and falls.
And its true! When we see in histories we found many examples of that type.
So with this reason its firm sure that 3rd war would be must.
Because now the balance between population and resources is totally damaged.

UN Failure:
For the enforcement of wars (and especially World Wars) only conflicts of nations are not becoming the only reason.
But when we look deeper in pre-war activities, we see the vital role is also one another. And that is the failures of World Associations.
In the first decade of 19th century there were blood clashes between Roman Church and Protestants, which had lost thousands of innadnt humans.
The patronage of these clashes were acted by Kings of Great Britain and France.
When millions people killed then Europe stood and made a lobby in which the 1st document named Magna Carta , signed. And wars stopped. The effected countries of Europe started re functioning the governments and for the betterment of people.

And its the great effort of Magna Carta, which'd born the League of Nations (LoN). Which had formed its total functions in 1934.
But as it loosen organizational dominancy The 1st World War happened, where millions people killed, crippled and been homeless.
And finally after many many treaties and pacts, it came to end.
But the crippled nations aimed to eachother to remain and unite under those treaties.
But within even 60 years those treaties and pacts as well LoN became weak to weaker and pipe hole of 2nd Great war opened.

And then United Nations formed in 1945 so replace the League of Nations, to stop wars between countries and to provide a platform for dialogue. It stated aims to facilitate cooperation in International law, security, economic development, social progress, human rights and achieving World Peace.
No doubt that for the betterment of whole humankind many treaties, charters, and foundations started their functions.
And its also remarkable task that many countries had got freedom.
But after 1990, the disintegration of USSR, the UN became house servant of USA.
And its fate resembles like ex Leagues.
And its now a days quite clear that UN exists in papers or engaged in dummy meetings, which has no results, or forceful voice.
After 1990 everyone knows about its disablity.
Simplys its said that there's nothing but the paper UN.

De Balance of Powers:
After the disintegration of USSR, many capitalist countries had initially felt happiness but none knew that now a Giant Ogre will make one man show to whole globe.
And surely everyone knows that its Uncle Sam!
At begining He tested his stereo policies on Gulf, and after ample success from their he drilled his laws on Nicaragua, Cuba, Iraq and finally his directions came to Afghanistan n Pakistan.
After 9-11, He moaned and gestured to the whole earthians that only he is sole protector from (his self made) terrorists, and only all countries should must obey to his rules.
And the whole mankind is now only seeing that they're helpless, hopeless.
Only in named democratic countries their rulers are imported, their wish is slaughtered.
But for much time?
Cry comes from even in Uncle's home.
This would be not stopped.
The guns would must be opened.
The balance should must be in normal.
The war for peace should must be come to true.
Its fate.

My dear fellows, these opinions are mine. Its not necessary that all these may be right.
I've skimmed out from different sources. May be I'm wrong.
So please join me and share your precious thoughts too.
I'll say warm welcome to all you! Thank you very much.