Why are Chadian troops withdrawing and where does this leave the force in Mali ?

Chadians today are enjoying a national holiday - as the country welcomes home some of their troops who had been fighting against Islamist forces in Mali. Nearly four months after a French led mission to oust militants linked to Al Qaeda, the troops were welcomed back at what seemed to be a lavish and unprecedented ceremony with many in the capital waving flags and shouting words of support for their forces. But why are they withdrawing now? To find out more I called up Chad expert Roland Marchal. BBC Africa first asked if the hoilday and welcome came as a surprise?