This is an especially tense time in the relationship between Sudan and Egypt, which has always been fraught. But as the geopolitical tectonics start to shift, both in Africa and the Middle East, the two neighbours are increasingly finding themselves on different sides of a number of crucial issues.
The most obvious dispute centres around the Halayeb Triangle, a 20 000 square kilometre area of land on the Egypt-Sudan border that is claimed by both countries. Egypt currently occupies the area, considered strategically important thanks to its Red Sea access, and has consistently refused Sudanese attempts to submit the matter for international arbitration.
In March this year, Khartoum reignited the issue by establishing a special committee to demarcate the Sudanese border, threatening to expel Egyptians caught on whatever they determined as the wrong side. The decision sparked outrage in Egypt.
'Neither the Egyptian people nor the armed forces will let go of a single centimetre of the Egyptian territory,' said Major General Kamal Amer, the head of the Egyptian parliament's National Defense and Security Committee.
Despite the feisty rhetoric, this land issue pales in significance compared to the water issue that now dominates regional politics.
Egypt and Sudan's responses to Ethiopia's Grand Renaissance Dam have been very different
Ethiopia is in the process of constructing its Grand Renaissance Dam, a mega infrastructure project on the Nile that will fundamentally alter how the river's water is distributed. Both Egypt and Sudan are heavily dependent on access to Nile water for agriculture and industry, and fear that Ethiopia will be able to cut off or reduce that access.
But their responses to the issue have been very different. After some initial concerns, Sudan has publicly embraced the dam, and signed deals with Ethiopia to benefit from its new hydro-electric capacity. Sudan has decided that its water security lies in friendly relations with Ethiopia.
Egypt has taken a different tack, demanding instead that Ethiopia halt construction and, at times, threatened war. It has sought to delay and undermine the new dam at every turn, despite Ethiopia's protestations that water flow will not be significantly affected. Efforts to address Egypt's concerns diplomatically have been largely unsuccessful: last month, the Middle East Monitor reported that the 14th round of trilateral talks between Egypt, Ethiopia and Sudan failed to resolve key issues.
Another bone of contention between the two countries is the ongoing civil war in South Sudan. Egypt is on President Salva Kiir's side, and has been repeatedly accused of arming his government and bombing rebel positions. Khartoum, on the other hand, is widely suspected of supporting Riek Machar's rebels by supplying weapons.
So far, efforts to repair the fraying relationship between Egypt and Sudan have been fruitless
Khartoum also suspects Egypt and South Sudan of supporting rebels in its own Darfur region, where Sudan has been waging a war against various militant groups for decades. In May this year, President Omar al-Bashir made the startling allegation that Egypt was supplying the rebels with military equipment. Although this allegation was swiftly denied by Egypt - 'Egyptian policy is constant, unchanging, and will not change; we do not interfere in the affairs of others,' claimed President Abdel Fattah el-Sisi, somehow maintaining a straight face - it has only exacerbated tensions.
Complicating the relationship further is that both Egypt and Sudan are caught up in Middle Eastern politics too. On Monday, Saudi Arabia and its allies all severed ties with Qatar, accusing the tiny Gulf state of supporting extremism in the region. Among those allies was Egypt, which has received billions in funding from Saudi Arabia since Sisi seized power.
Although Saudi Arabia has been aggressively courting Sudan in recent years, Khartoum remains on the fence, having welcomed Qatar's emir on a state visit last year. That visit caused its own bizarre controversy: when the emir's mother, Sheikha Mozah bint Nasser, visited the ancient pyramids in northern Sudan, Egyptian media reacted furiously, accusing Qatar of promoting Sudan's pyramids while ignoring those of Egypt.
The incident, while frivolous, underscores just how sensitive both countries are too any kind of perceived slight. Qatar has also played a leading (albeit largely unsuccessful) role in mediating between Bashir's government and various rebel groups, holding several rounds of peace talks in Doha.
This puts Sudan in an awkward spot: Saudi Arabia's diplomatic offensive against Qatar leaves it caught between two key allies, and - once again - at odds with its neighbour Egypt. In a bid to resolve this dilemma, Sudan has offered to lead 'reconciliation' efforts between Saudi Arabia and Qatar, although no one has yet shown any interest in taking up this offer.
Egypt and Sudan are caught up in African and Middle Eastern politics
All these diplomatic and geopolitical considerations are beginning to have a practical impact.
Last month, Sudan took the extraordinary step of banning all agricultural and animal imports from Egypt - never mind that imports from Egypt constitute more than 5% of Sudan's total imports. Egypt has slapped a ban on Sudanese journalists from entering the country, while Sudan now requires all Egyptian males between the ages of 18 and 50 to obtain visas prior to arrival.
So far, efforts to repair the fraying relationship have been fruitless. Sudanese foreign minister Ibrahim Ghandour was in Cairo this week to meet his counterpart, Sameh Shoukry, as well as Sisi. While both sides said all the right things publicly, Egyptian media reported that it was a different story behind the scenes, where diplomats failed to resolve their differences.
The truth is, for all their long history, Egypt and Sudan have increasingly little in common. Both in Africa and in the Middle East, their respective trajectories are leaving them on opposite sides of the fence. It's a tension that this part of Africa, already characterised by endemic conflict, could do without.
Simon Allison, ISS Consultant