The United Front for Change (Fuc) is a coalition of several smaller armed groups and army deserters, many of whom fought to place President Idriss Deby in power in 1990.
Under the leadership of 35-year-old Mahamat Nour, the alliance is well armed and says it wants to get to N'Djamena before elections due at the beginning of May and before the start of the rainy season.
For the past six months, Nour has been building up his forces in bases in Darfur, the lawless Sudanese region across Chad's eastern border, from where both Mr Deby and before him ex-President Hissene Habre launched their coups.
This has infuriated Mr Deby, whose government has accused Sudan of backing the Fuc.
But as Sudan battles its own rebels in Darfur - drawn from the same Zagawa ethnic group as Mr Deby - it hurls similar accusations at Chad.
Tactics
For Mr Nour, this fallout of former allies has been timely and enabled him to plan strategically.
Chad: Ripe for a coup?
The BBC French Service journalist Mahamat Adamou says Mr Deby's forces are mostly concentrated in the east and the recent rebel raids in the centre and south of the country have caught the military on the back foot.
The rebel alliance claims its intention once it reaches the capital is to organise a national forum that will lead to a transitional government and on to democratic elections.
Fuc accuse Mr Deby of being a dictator whose corrupt rule is to blame for the mismanagement of the economy.
The largely arid country became an oil-exporter in 2003 but after decades of civil war there is little infrastructure and widespread poverty.
Family feud
Mr Deby, who won Chad's first elections in 1996, is standing again in next month's polls.
The constitutional two-term presidential limit was amended to allow him to seek another term in office.
Analysts believe it is anger at this move more than anything else that has fuelled the rebellion and even driven Zagawa family members into rebel arms.
Another rebel group called Scud - not part of the Fuc coalition - is headed by Mr Deby's powerful twin nephews.
The great danger if this offensive is successful, says Mahamat Adamou, is that these two groups or the Fuc coalition itself may fall out.
For ordinary Chadians it would be like returning to 1979, when at least 12 armed groups took N'Djamena and attempted to share power.
After a few months they started fighting amongst themselves leaving Chad facing further unrest and years of civil war.
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©
~RS~q~RS~~RS~z~RS~08~RS~)