Why US,UK did nothing after spotting the Chibok girls in Sambisa forest (Must Read)

It has been revealed  that both the British and United States (U.S.) governments knew where some of the schoolgirls kidnapped by Boko Haram members from Chibok but felt powerless to launch a rescue mission, it has been reported.A former British High Commissioner to Nigeria, Dr Andrew Pocock, has said that a large group of the missing girls were spotted but the governments felt any rescue attempt was risky.
He told The Sunday Times:

“A couple of months after the kidnapping, fly-bys and an American eye in the sky spotted a group of up to 80 girls in a particular spot in the Sambisa forest, around a very large tree, called locally the Tree of Life, along with evidence of vehicular movement and a large encampment.
“A land-based attack would have been seen coming miles away and the girls killed, an air-based rescue, such as flying in helicopters or Hercules, would have required large numbers and meant a significant risk to the rescuers and even more so to the girls.
“You might have rescued a few but many would have been killed. My personal fear was always about the girls not in that encampment — 80 were there, but  250 were taken, so the bulk were not there. What would have happened to them? You were damned if you do and damned if you don’t.”