Rebels launched a major assault on Chad's capital, N'Djamena, in early February briefly seizing control of large parts of the city before being pushed back.
The crisis could have major implications for attempts to end the conflict in neighbouring Darfur, since Sudan's government is accused of backing the rebels in Chad.
The BBC's Stephanie Hancock, who has covered Chad for the past two years, explains.
Q: Who are the rebels?
The rebels who attacked N'Djamena are a new alliance of three main groups, who joined forces just two months ago.
The largest group is UFDD (United Force for Democracy and Development), which is led by Mahamat Nouri.
He is a former member of President Idriss Deby's government and was working as Chad's ambassador to Saudi Arabia when he defected to the rebellion two years ago.
He is a Gorane, the same clan as Chad's former President Hissene Habre, whom Mr Deby ousted in a coup 17 years ago.
The second main group is RFC (Rally of Forces for Change), led by Timane Erdimi, President Deby's former chief of staff.
His role as a rebel leader is controversial as not only is he a Zaghawa, the same clan as Mr Deby, but he is also the president's own uncle.
The third rebel group is UFDD-Fondamentale, a splinter group of UFDD, currently headed by Abdelwahid Aboud.
Why are they fighting?
The rebels, as well as President Deby's political opponents, say that his rule has been both violent and corrupt.
They accuse him of favouring members of his Zagawa clan, who make up less than 3% of Chad's population, above other citizens.
Mr Deby has packed much of his government and the army with members of his clan, or other trusted allies.
Critics say he has rigged elections to enable him stay in power, and after Chad started pumping oil in 2003, he has allegedly embezzled much of the country's oil riches.
The tide began to turn against President Deby when, against the wishes of most Chadians, he changed the country's constitution to run for an unprecedented third term in office.
This prompted a wave of desertions from the Chadian Army and sowed the seeds of today's rebellion.
Didn't the rebels nearly succeed before?
President Deby was almost toppled in April 2006, when rebels again attacked the capital.
But the attack was poorly co-ordinated, and led by only one rebel group, and once they got to N'Djamena, the fighters were simply outnumbered and overpowered.
This time things were much better planned.
Three rebel groups have been able to work as one unit, and more than 2,000 men took part.
They also wrong-footed the Chadian Army, which was expecting an attack in the east of the country, where the rebels have their bases.
Using the element of surprise, the rebels were able to cross more than 1,000km (620 miles) of terrain virtually unchallenged, and eventually enter the capital with surprising ease.
Is Sudan backing the rebels?
Khartoum has repeatedly denied backing the rebels, but it is well known that the Sudanese government does support them.
It provides not only weapons and vehicles, but also uniforms and medical supplies.
Sudan is backing the Chadian rebels to counter the threat that it faces from its own rebels in Darfur.
Many Darfur rebels are from the same ethnic group as President Deby, and ever since their uprising began, he has offered them support.
The Darfur rebels enjoy a free reign in Chad, and are so close to President Deby that it is not uncommon for them to fight alongside the Chadian Army when it needs help.
Sudan has reacted to this by, in turn, arming Chadian rebels.
What are the possible implications for Darfur?
In the short term, aid organisations will be anxious about the fate of 250,000 Sudanese refugees in eastern Chad, as well of tens of thousands of Chadian displaced people who have been affected by the spill over of violence.
If insecurity continues, getting in supplies will become increasingly difficult, and with soldiers and rebels now concentrated miles away in the capital, a security vacuum could put aid workers and the displaced at risk.
In the long term, because the Darfur crisis is intimately linked to Chad-Sudan relations, the outlook is uncertain.
As long as Chad and Sudan continue to wage war on each other, the Darfur crisis will be almost impossible to resolve.
What is likely to happen to the EU mission to eastern Chad, to protect refugees from Darfur?
Chad says the timing of the current assault is an attempt to stop EU soldiers being deployed.
But despite the violence of the past few days, the EU insists its planned peacekeeping mission will go ahead, and says the deployment has simply been delayed by four weeks.
The reality is more complicated.
Should President Deby be toppled, it is not known whether Chad's new leader would still accept foreign troops, and any new regime could put a stop to deployment.
If Mr Deby stays in power, but security remains dire, it may be too risky to deploy foreign troops as they could potentially get caught up in a war they could neither understand nor control.
France, which spearheaded the mission and is providing most of the troops, risks huge loss of face if the deployment is cancelled.