By Mark Doyle BBC World Affairs correspondent

The desert state of Chad borders other unstable countries

Before the days of air travel, land routes for armies mattered and the French saw that the old desert camel routes, used by salt and gold traders, could be used to their advantage.

The French never quite managed a land link between all their conquests with Chad at the centre. The British got in the way of that by grabbing Sudan and the Italians took Libya.

But today Chad is still in a strategic position. It sits at the centre of several theatres of war including Sudan and the Central African Republic, and also has borders with other unstable countries.

Fighting in the east

The recent fighting between Chadian government and rebel forces has been most intense in eastern Chad, near the border with Sudan's Darfur region where the United States says the Sudanese government has committed genocide, using Arab tribes to attack ethnic Africans.

The ethnic makeup of eastern Chad is similar to that in Darfur and the fighting in Sudan has spread into Chad.

The Chadian town of Abeche, near the Sudanese border, has throughout Chad's turbulent history often been the first significant place to fall to rebels before an attack on the capital, N'Djamena.

The former Chadian rebel Hissene Habre, for example, fought for and took Abeche in the early 1980s before going on the take the capital and hold onto power as president for almost two decades.

The current President, Idriss Deby, also launched attacks, on Hissene Habre, from the Abeche area.

When President Deby denounces Sudan's alleged backing for the current rebels, he may be talking from a position of some authority because his original invasion, in November 1990, is alleged to have been launched from Sudan - although the Sudanese government denied involvement.

The current Deby government says it retook Abeche over the weekend after it was briefly held by insurgents.

The Chadian military have had to reclaim towns from rebels

There are confusing reports about where the rebels may now be. This is understandable because the terrain between Abeche in the east and the capital in the west is largely uninhabited desert.

Occasionally information about rebel movements is leaked from the French embassy in N'Djamena as they inform French expatriates of the level of danger they may be in.

The French have air patrols and probable access to spy satellites - although sandstorms can sometimes obscure their images.

France has an estimated 1,000 soldiers in permanent bases in N'Djamena and Abeche. They are a hangover from the earlier colonial ambitions and a tool of current policy.

They provide logistical and intelligence support to President Deby's army - and the rebels say they do more, actively supporting the regime in N'Djamena.

'Fragile lifeline'

Chad has several other highly sensitive borders. To the north is Libya where there have been historic disputes over an area called the Aouzou Strip, and to the south is the unstable Central African Republic (CAR).

Thousands of people have been displaced by fighting in Chad

Some 46,000 refugees from the ethnic and political conflict in the CAR are currently in southern Chad.

In recent years Chad has also acquired a new strategic significance as its crude oil has come on stream. The oil money has undoubtedly spurred the activities of arms dealers selling equipment to both the government and rebel sides.

The fighting in the east has threatened what the United Nations calls its "fragile humanitarian lifeline" to Sudanese refugees and war-displaced Chadians in eastern Chad.

The UN refugee agency, UNHCR, runs a string of camps inside Chad for refugees from the conflict in Sudan and Chadians displaced by the conflict which has spilled over into Chad.

The sun-blasted desert camps of eastern Chad are in one of the most inhospitable places on earth. There is hardly any natural cover - such as trees - from the incredibly hot sun, and little water.

All food supplies have to be trucked or flown in by aid agencies. The camps hold some 220,000 Sudanese and around 90,000 displaced Chadians.

Over the weekend, the UNHCR said the humanitarian lifeline to these people was "very, very fragile and we fear that continued violence in the region could easily sever it, jeopardising the lives of thousands of Darfurians and Chadians".