General Sir Richard Dannatt said the decision to deploy a task force to Helmand in 2006 at a time it was facing worsening insurgency in Iraq represented a "perfect storm" for the Army.
Giving evidence to the official Iraq Inquiry, he disclosed that he had written to the then defence secretary Des Browne to warn him that morale was "fragile".
According to Sky News, he advised Browne that it could lead to a potentially devastating exodus of personnel from the service.
"We could see that perfect storm coming to fruition in about the middle of 2006 and I would contend that it did," General Dannatt said.
"You can run hot when you are in balance and there is enough oil sloshing around the engine to keep it going. When the oil is thin, or not in sufficient quantity, the engine runs the risk of seizing up," he told the inquiry," he added.
General Sir Mike Jackson said: "My biggest concern was that that fragility could be turned into a sharp rise in exits from our trained manpower akin to going over a cliff edge.
"Once your manning has begun to plummet we would have been in all kinds of trouble trying to man two operations with units that were not fully manned. That would have spiralled into something of a nightmare," he added. (ANI)