Chad has been plagued by violence for more than 40 years, with various tribal and political factions struggling for power.
|Chadian rebels have made several attempts to topple the government of Idriss Deby [AFP]
Since 1975, every government in the capital Ndjamena has come to power as the result of an armed rebellion, several of them originating in the east of the country.
The latest such attempted rebel offensive was apparently crushed by government forces earlier this month, as air raids killed 220 fighters and destroyed 100 of their vehicles, according to officials.
Chad took the mopping up operation that followed as far as the border with Sudan, where the government says the rebels are based and receive material support from the Khartoum government.
Idriss Deby, the Chadian president, is from the east himself and, in 1989, launched an armed campaign against the then government from across the border in Sudan's western Darfur region.
Initially, the Sudanese backed Deby's bid for power and he eventually toppled Hissene Habre, the former Chadian leader, one year later.
However, after several years of good relations, Deby fell out with the Sudanese government over the Darfur conflict, blaming Khartoum for the killing of members of his Zagawa ethnic group who straddle the Chadian-Sudanese border.
Ethnic ties do not, however, tell the whole story as the Zagawa have never been united in one tribal or political entity.
In Darfur, they are divided between the factions of Sudan Liberation Movement (SLA) of Minni Minnawi and the Justice and Equality Movement (Jem) led by Khalil Ibrahim.
The Zagawa of Chad are split between those loyal to Deby and others supporting the rebel groups in the east.
Chad's rebels have failed several times to reach Ndjamena and topple Deby, but before the May 4 offensive they claimed that they were more prepared than ever.
Preparations had been under way for five months and eight groups had been brought together under the banner of the Union of Resistance Forces (UFR).
They chose the leader of the Rally of Democratic Forces, Timane Erdimi, a nephew of Deby, as commander-in-chief. The move was designed to sway some of Deby's generals from the same tribe and thus weaken his support among his own military.
The government success, therefore, led it to claim not just a decisive victory over the UFR, but also over Khartoum, which it accuses of directly orchestrating the attack.
Sudan has categorically denied any involvement, but it is widely believed that Khartoum provides rear bases and military support to some Chadian factions.
For its part, Ndjamena is thought to provide backing for the Justice and Equality Movement in its conflict with Sudanese government forces.
|In 2008, rebels reached Ndjamena and besieged the presidential palace [File: AFP]
The ongoing tensions between the two neighbours began to reach a climax in April 2006, when Chadian rebels battled their way to the outskirts of Ndjamena leaving hundreds of people dead.
Chad cut diplomatic ties with Sudan, accusing it of backing the rebels.
A similar incident in February 2008 saw rebels besiege the presidential palace and nearly topple the government. Deby needed French military assistance to stay in power.
One month later, the two countries signed a peace deal following negotiations brokered by Senegal.
The accord was, however, short-lived as relations came under strain yet again just three months later when Jem rebels advanced as far as the Sudanese capital.
It was the first time that a Darfuri group had managed to attack Khartoum.
The Sudanese government saw the offensive as a clear act of aggression by Chad and diplomatic ties between the two countries were severed again.
This month's offensive in Chad, and the subsequent tit-for-tat allegations, came shortly after the two sides signed the latest in a series of agreements supposed to bring hostilities to an end.
Now the prospect of a final peace deal, scheduled to be signed by Deby and Omar al-Bashir, his Sudanese counterpart, in Libya this month has been thrown into doubt.
It is highly unlikely that Sudan and Chad can ever be fully reconciled without a resolution to the conflict in Darfur, but there are other factors fuelling Chad's political instability.
Libya and France have both played a part in previous armed rebellions, supporting efforts to topple governments opposed to their interests or backing Ndjamena when a friendly leader is in power.
Libya shares ethnic ties with Chad, while France views the former colony of Chad, which exports more than 200,000 barrels of oil a year, as a strategic entry point into oil-rich central Africa.