The border region between Chad and Sudan has become the focus of numerous wars and is the site of one of the world's most serious humanitarian crises.
Cross-border fighting, as well as internal civil wars in both countries, have caused some 2.4m people to flee their homes in recent years.
The two countries have become "brother enemies" and they are neighbours with many common traits.
Millions of people have been displaced by the warring parties
Both regimes came to power in military coups; both oppress their civilian opponents and both have recently discovered and exploited significant quantities of oil which have made their elites rich.
About two million people have been displaced by rebel activity and government repression in the Sudanese region of Darfur, and around half a million Chadians and Sudanese refugees are also homeless in eastern Chad.
I recently visited both sides of the frontier, accompanying a fact-finding tour by the United Nations Security Council which touched down in Fasher, one of the main cities in Darfur, and in Chad's eastern capital Abeche.
The diplomats also visited camps for refugees and displaced people in both countries.
Everywhere we went we saw evidence of the heady mixture of military might and dispossessed people which typify war zones.
In Fasher, the white UN plane we travelled on shared a runway with powerful Russian-made Hind helicopter gunships and Antonov transport planes used by the Sudanese Air Force
In Abeche, we saw the new base of a European military refugee protection force, Eufor, as well as meeting a variety of Chadian soldiers, militiamen and armed police.
Everywhere we saw the pathetic shelters of the war-displaced and the refugees who suffer at the hands of the armies and rebels of the region.
Recent fighting in the border area - the latest involved Chadian rebels fighting in areas north and south of Abeche with government soldiers - has exacerbated the already acute humanitarian crisis in the area, with more people fleeing and the movements of aid workers restricted.
Eufor is dominated by French troops but also includes Irish and other European soldiers.
It has been mandated since early this year to protect the homeless Chadians and Sudanese living in camps along the Chad side of the border.
It is an extremely sensitive mission because Chad and Sudan regularly accuse each other of backing the others' rebels.
Independent observers say the mutual accusations are probably true.
Some analysts (including some close to the Sudanese government in Khartoum) have accused Eufor of harbouring a bias towards protecting the Chadian government in N'Djamena.
They say Eufor's French component is allied to the French military contingent which has a separate bilateral defence pact with N'Djamena.
But Eufor says it is neutral, and others say France only dominates the force because no other European nation would supply sufficient troops and equipment necessary to protect the Sudanese refugees and internally displaced Chadians.
Tit for tat
The wars in Chad and Sudan are deeply intertwined.
Chadian rebels almost took N'Djamena in February before they were repulsed by forces loyal to the embattled Chadian President Idriss Deby.
The French-dominated EU peace force is regarded by some as biased
Mr Deby accused Sudan of being behind the attack.
Ironically, President Deby is reported to have received Sudanese support for his original coup d'etat against former Chadian President Hissene Habre in 1990.
Authors Alex de Waal and Julie Flint report in their recently published book Darfur: A New History of a Long War that Khartoum had several motives for backing Mr Deby, including hoping for a friendly and indebted government on its vulnerable western border.
They may have got that in the early 1990s, but as Mr Deby's power base has narrowed over the years, with defections to rebel ranks, he has accused Sudan of backing these rebels and relations between the two capitals have hit a new low.
A few months after the February raid on N'Djamena, a near-mirror image event took place.
Sudanese rebels reached the edge of Khartoum, almost crossing a bridge from the twin city of Omdurman which leads into the city centre itself.
Khartoum subsequently mounted a propaganda display of military items it said had been captured from the rebels.
The captured military vehicles and uniforms, the government said, proved Chadian involvement in the attack.
African and Arab ethnic groups straddle the common border between Chad and Sudan which was drawn up more for the convenience of French and British colonial powers than out of any long term concern for the local populations.
This has contributed to many apparent contradictions.
Chadian troops celebrate a victory against Sudanese-backed rebels
Ethnic and tribal groupings are still powerful in both Chad and Sudan.
Historically, one of the strongest border tribes, in military terms, is the Zagawa.
Mr Deby is Zagawa, but so are many of his opponents who have defected to Chadian rebel ranks.
The Zagawa are also significant players in the Sudanese rebel groups opposed to the Sudan's government.
And to add to the complications a key Zagawa warlord, Minni Minnawi, was recently co-opted from the Sudanese rebels to become a presidential adviser.
No-one ever said these wars were simple.