"Western political leaders may have no appetite for deeper intervention. But as history has shown, we do not always choose which wars to fight – sometimes wars choose us."
He added: "Military planners have a responsibility to prepare for intervention options in Syria for their political masters in case this conflict chooses them.
"Preparation will be proceeding today in several Western capitals and on the ground in Syria and in Turkey.
"They will however have grave reservations over the consequences and the cost of intervention as well as the geopolitical implications.
Uppermost in Western government minds will be securing the five different chemical agent manufacturing plants and dozens of storage sits in hardened bunkers.
The paper argues "perhaps as many as 75,000 troops" will be required to safeguard and safely dispose of Syria's substantial chemical weapons stockpile to ensure there is no risk of terrorists using them.
Michael Codner, RUSI's Director of the Military Sciences, wrote that a starting point for calculations of full scale intervention would be at least 300,000 troops.
The paper concludes: "Western governments which have long worked for Assad's departure should now begin to fear what may lie in store. For, instead of imploding as other Arab countries did when they were gripped by revolutions, Syria will explode, disgorging its troubles across the entire Middle East, with potentially catastrophic consequences."
