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* African, western allies are left scrambling to catch up
* Dislodging al Qaeda from north Mali seen as "massive" task
* Islamists expected to dodge battle, seek hit-and-run war
* Intervention could lead to "messy" counter-insurgency
By Pascal Fletcher
JOHANNESBURG, Jan 13 (Reuters) - France has rushed to help Mali block a push south by Islamist rebels from its desert north but a U.N.-backed intervention plan to dislodge al Qaeda and its allies faces a tough enemy and terrain and could still take months - if it succeeds at all.
The original timetable for the AFISMA intervention force of 3,300 West African troops with western logistical, financial and intelligence backing did not foresee deployment before September, to allow time for full preparation.
But this has now been accelerated by the rushed French response to a plea for help by Mali's government, after mobile columns of Islamist fighters last week threatened the central garrison towns of Mopti and Sevare, with its key airport.
With French jets and helicopters hitting Islamist positions in Gao and other rebel-held towns, West African regional grouping ECOWAS is now scrambling to get its troops onto the ground in Mali, raising questions about the long-term mission.
"Rushing into the intervention right now provides a shaky ground for the mission," said Martin van Vliet, a researcher at the African Studies Center at Leiden in the Netherlands.
"But doing nothing would have been a bad option as well," added van Vliet, who has also written about Mali for the U.S. Military Academy at West Point's Combating Terrorism Center.
Most experts agree that taking on several thousand fighters - a mix of Tuareg rebels, Islamists and foreign al Qaeda jihadists - in an inhospitable desert and mountain battleground the size of Texas is a huge military challenge.
"A massive, massive undertaking," was how one of the U.S. State Department's leading officials for Africa, Reuben Brigety, assessed the task facing Mali, ECOWAS and their western allies when he spoke at London's Chatham House in late October.
"That is incredibly difficult terrain; it's a vast expanse. It will take a long time to take and hold," said Brigety, a former soldier, who added the full-scale Mali intervention would be "hugely expensive" and would last months, not weeks.
The United States and United Nations have stressed the internationally backed Mali mission should be "African-led, African-owned", and accompanied by a political process to pacify the split Sahel state, once a beacon of democracy and stability.
But with French special forces and aircraft taking the fight to the Islamists, and French Defence Minister Jean-Yves Le Drian declaring France "at war against terrorism wherever it is found", the Mali operation is a French-led show for the moment.
French President Francois Hollande has taken pains to cloak his country's swift military action in the mantle of the Dec. 20 U.N. Security Council resolution that authorised the deployment of the larger AFISMA force.
This same resolution set key "benchmarks" for Mali, including a political agreement and verification of the training - including human rights courses - and of the operational readiness of the battered Malian army and the African intervention force. But none of these have been completed.
"The Malian army is still in complete disarray," said van Vliet, pointing to its loss last week of key locations including Konna, since recaptured in the French-backed counter-offensive.
"GROUND GAME" IS KEY
French state-of-the-art Rafale fighter jets on Sunday pounded rebel positions around Gao, the largest city in the Islamist-held north, as Paris deployed several hundred troops in the southern capital Bamako and in central Mali.
"My first impression is that this is an emergency patch in a very dangerous situation," said Gregory Mann, associate professor of history at Columbia University, who specialises in francophone Africa and Mali in particular.
Mann said the "ground game" would be key - just how quickly and effectively the French-backed Malian army and its ECOWAS allies could follow up on the air strikes to try to retake the main rebel strongholds in the north - Gao, Timbuktu and Kidal.
"Right now, we have cause for relief, but not optimism," he added.
The northern towns fell under Islamist control after a March military coup in Bamako triggered a Tuareg-led rebel offensive that seized the north and split the West African nation in two.
Security experts say the Islamist rebel forces, which include a hard core of experienced al Qaeda in the Islamic Maghreb (AQIM) fighters, are likely to shun a head-on battle with the joint ECOWAS/Malian army contingent, which is expected to exceed 6,000 soldiers when eventually fully deployed.
Instead of trying to defend Gao, Timbuktu and Kidal, the Islamists are expected to melt back into the trackless deserts and rocky mountain fastnesses of north Mali, such as the Adrar des Ifhogas range near the northeast rebel-held town of Kidal.
Allied to Algerian-led AQIM are the Arab-dominated MUJAO faction and Malian Tuareg rebels of the Ansar Dine group led by veteran Tuareg rebel and politician Iyad ag Ghali.
"A ground intervention is unlikely to result in significant defeats for AQIM or MUJAO," Richard Jackson, Deputy Director of Violent Risk Forecasting at UK-based Exclusive Analysis consultancy wrote in a January report on West African jihadism.
Exclusive Analysis estimates the combined strength of AQIM, Ansar Dine and MUJAO together at up to 3,000 fighters.
"Due to rebel strength and the size of the occupied territory, military action is likely to be prolonged," Jackson wrote in the report.
AQIM, described by the head of U.S. Africa Command General Carter Ham as "well financed and very well armed", likes to operate with mobile columns of 4x4s and pickup trucks, some mounted with heavy weapons, a staple of desert warfare in the African Sahel, from Mauritania to Chad and Somalia.
But these are vulnerable in open desert and scrubland to attack from French Gazelle helicopters and Rafale fighters.
"It remains to be seen whether the Islamists have anti-aircraft defence weapons," said Mann, adding he had seen reports they had anti-aircraft cannon and even surface-to-air missiles.
Van Vliet said that even if the internationally backed African force succeeded in retaking Gao and Timbuktu, and the remoter Kidal, the second phase of the Mali insurgency would likely involve "messy counterinsurgency", with troops facing hit-and-run attacks from an elusive, shifting enemy.
"Once the intervention happens, there has to be a plan for after it. You can't just intervene to get rid of AQIM and then leave again, otherwise it will just fall into disarray again," said one Western diplomat, who asked not to be named.
With AQIM and Ansar Dine already threatening reprisals, there are heightened fears of Islamist militants targeting French and Western nationals and interests across West Africa, and even further afield in Europe.
U.S. AFRICOM chief Ham pointed last year to what he called "linkages, networks and coordination" between AQIM in the Sahel, the al Shabaab Islamic militant group in Somalia and the violent Boko Haram insurgency in top African oil producer Nigeria.
Any hope of completely eradicating al Qaeda in Mali would require shutting off the huge porous border of the north that juts into the vast Sahara - an almost impossible task. It would also need the cooperation of neighbours Algeria, Mauritania and Niger.
Some analysts said perhaps the best measure of success the intervention plan could hope for would be the recapture of the main northern urban areas and containment of the insurgents.
"If in two years in the future, the people of Gao, Timbuktu and Kidal are able to go about their lives peacefully, I'd say that was a huge success, even if the deep desert north was still largely out of government control," said Columbia's Mann. "It always has been." (Additional reporting by John Irish in Paris and William Maclean in Dubai; Editing by Daniel Flynn and Will Waterman)