All students of geopolitics are familiar with the legend that Egypt has privately warned all the governments upstream on the Nile that it will start bombing if they build dams on the river without its permission.
The truth of that story is about to be tested.
Last month, Ethiopia started diverting the waters of the Blue Nile in order to build the Great Ethiopian Renaissance Dam, a $US4.7 billion ($NZ5.8 billion), 6000MW hydro-electric project that is the centrepiece of the country's plan to become Africa's largest exporter of power.
Egypt instantly objected.
''We have a strong legal case to insist that our share of the Nile water is preserved,'' an anonymous government source said - but he did not mention bombers.
Egypt depends utterly on irrigation water from the Nile to grow its food. Even now there is not enough (it already imports almost 40% of its food), and Egypt's population is still growing fast. If the amount of water coming down the Nile diminishes appreciably, Egyptians will go hungry.
A treaty signed in 1929 gave 90% of the Nile's water to the downstream countries, Egypt and Sudan, even though all the water in the river starts as rain in the upstream countries: Ethiopia, Uganda, Kenya and Tanzania.
It seemed fair at the time: the 20 million people in the downstream countries depended heavily on irrigation, while the 27 million in the upstream countries had plenty of rain-fed land and hardly irrigated at all.
Things have changed since then.
According to the International Data Base of the United States Census Bureau, there are now six times as many people in the Arabic-speaking countries downstream, and eight times as many people in the African countries upstream.
Egypt is using all of its share of the water - and the upstream countries are starting to use the water for irrigation, too.
The Great Ethiopian Renaissance Dam is the first real test of Egypt's tolerance for upstream dam-building. The reservoir will take 63 million cu m of water to fill; Egypt's annual share of the Nile's water is 55.5 million cu m.
So even if Ethiopia takes five years to fill the reservoir, that will mean 20% cuts in the water Egypt receives from the Nile for five years. And even after that there will be a large annual loss to evaporation.
This dam is just the start. Ethiopia plans to spend a total of $US12 billion on dams on the Blue Nile for electricity and irrigation, and Uganda is negotiating with China for financing for a 600MW dam on the White Nile. More dams and irrigation projects will follow - and the upstream states are in no mood to let Egypt exercise its veto under the 1929 treaty.
That treaty was imposed when all the countries involved except Ethiopia were under British rule.
In 2010, six upstream countries (including Burundi and Rwanda) signed a Co-operative Framework Agreement to seek more water from the Nile, effectively rejecting the colonial-era treaty and demanding that Egypt relinquish its veto and accept a lower water quota.
That is not going to happen. Mohammed Allam, Egypt's minister of water resources under President Hosni Mubarak when the upstream states signed their agreement three years ago, warned that ''Egypt reserves the right to take whatever course it sees suitable to safeguard its share''.
His country sees the matter as a national security issue, Mohammed Allam said: ''Egypt's share of the Nile's water is a historic right that Egypt has defended throughout its history.''
The post-revolutionary Egyptian Government under President Mohammed Morsi cannot afford to be less firm in defending Egypt's interests.
The issue will probably be kicked down the road for a couple of years, because the Great Ethiopian Renaissance Dam will not be completed until 2015 at the earliest. But there is big trouble for Egypt (and Sudan) further down the road.
By 2025, a dozen years from now, Egypt will be trying to feed 96 million people, which would be very hard even with its existing giant's share of the Nile's water and all its current food imports.
The countries that signed the Co-operative Framework Agreement will have 300 million people, so by then they will also be extracting very large amounts of water from the Nile Basin for irrigation.
Without that water, Egypt's only options are beggaring itself with massive food imports (until the foreign exchange runs out altogether), or famine. Unless, of course, it decides on war - but its options are not very good on that front, either.
Not only are the upstream countries a very long way from Egypt but they will have strong support from China, which is financing most of the dams they are now building or planning.
Egypt, by contrast, has repudiated its former American ally, and may find that the US is reluctant to re-engage even if the Government in Cairo can overcome its own distaste for Washington. Why would the US want a confrontation with China over Egypt?So there probably won't be a war. And Egypt will probably face an apocalyptic food shortage in 10 or 15 years.
• Gwynne Dyer is an independent London journalist.
