We all think we know why wars are fought.
Whether it's the devastation in Syria, armed skirmishes in Africa or Russia's expansionist leanings, armed conflicts are usually seen as falling into one of very few categories; capturing territory, a political ideology attempting to dominate another or, simply, for a country to get its hands on oilfields.
But, according to one theory, whatever the stated reason for most wars, they actually come down to one reason. Or rather, one resource, which is all around us.
And with stores of this resource dwindling in some parts of the world, things could be about to get a lot worse with a potential future flashpoint being between two of the world's nuclear armed superpowers - India and China.
Alok Jha, a British journalist with a background in physics, will speak at this weekend's Festival of Dangerous Ideas at the Sydney Opera House about the role water has played in a multitude of conflicts including both the Arab Spring and the civil war that has engulfed Syria.
"The Roman empire and the Persian empire would go to war for access to water and would live or die by that," Jha says. "That doesn't happen as blatantly anymore, it's much more subtle."
Part of the problem, he argues in his new book The Water Book, is we've managed to hide water from view and have forgotten its importance.
Two million deaths
Conflicts concerning strategic bodies of water are nothing new. Indeed, a series of skirmishes in the 1960s between Israel, Syria and Lebanon about freshwater allocations from the Jordan Valley was called the 'water war'.
But Jha argues that far more conflicts have water at their core.
"There are conflicts and skirmishes all the time and they are often not described as a water war, sometimes people might fight over a bit of land or it may manifest as a trade dispute but underlying all of that is access to water.
"In the Middle East there are constant battles over (water) but it's at a very low level and sometimes internal (to the country)," he says.
"The Arab Spring was exacerbated by failed crops. Syria, right now, is largely political and it's about dictatorships and war but it's exacerbated because of water shortages."
The spark for the genocide in Sudan's Darfur region, which saw two million deaths and the country divided in two, is credited as the populated and parched north of the country looking to get its hands on water from the lush but culturally distinct south.
"If you do not have access to water, it's not just hard to have a shower in the morning, you can't do anything, you can't grow crops, you have no clothes, nothing works," he says.
If climate change continues, as scientists predict, there will be even less water in the Middle East, sub Saharan Africa, the southern US, the Mediterranean and, he says, Australia. And as the water moves, so will the people.
"If everything in a country dries up the people will look across international boundaries for jobs, food, for home. Richer countries will have to work out how to absorb these people.
Reasons for migration might seem diverse now, from looking for a better life to escaping conflict, "but in 20 years, if you look back you'll see it was a water-led migration," says Jha.
Conversely, massive global disruption could also occur because of too much water. Rising sea levels could swamp major world centres, like New York, London and Tokyo.
China and India
In the short term Jha said oil was still a more obvious cause of conflict. "You cannot run an economy without it. If oil resources run out a country will stop within a week, whereas with water every country has a natural resource of it and can eke it out a bit longer and it doesn't feel like it's a problem until it's too late."
One very big problem could be a future war about water between the world's two largest nations.
"China wants to build lots of dams near the Himalayas which will cause problems for India.
"India is desperate for water because it's polluted a lot of its own and the glaciers that feed the Ganges river are shrinking rapidly but China is richer and has the ability and technology to move water around, build power stations and divert rivers."
India and China are both nuclear armed, share a border and have had frequent clashes across their long and remote mountain frontier in the past.
Jha said China and India would be "crazy" to have a dispute over water but, "If one guy says I need this water here and the other one says no, it's about who gets it".