Britain will find it increasingly difficult to avoid taking part in action in Syria, a former Army commander warned today.
Colonel Richard Kemp, who led UK forces in Afghanistan, said the escalating civil war meant it was more likely that western governments would intervene to stop the bloodshed spreading to neighbouring countries.
Last week President Bashar al-Assad's regime was rocked by the assassinations of two defence ministers and a series of military defections.
Col Kemp said today: "Whether or not Assad falls, the question of military intervention will remain a live issue.
"External intervention has been under way in Syria for months, with Russia arming the regime.
"At the same time Saudi Arabia and Qatar, with US and Turkish facilitation, have been arming and funding the opposition; and this covert support has been substantially responsible for the progress opposition forces have made in recent weeks.
"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."
Options could include arming opposition fighters, diverting cash to rebel authorities or joining a coalition for military action.
Prime Minister David Cameron ordered British forces into action last year in Libya, with RAF jets enforcing a no-fly zone, Royal Navy warships firing missiles off the Libyan coast and special forces helping rebels topple leader Muammar Gaddafi.
Col Kemp's comments came in A Collision Course for Intervention, a paper published by the Royal United Services Institute as tension is ramped up along Syria's borders, sparking fears of wider regional conflict.
Foreign Secretary William Hague last week criticised China and Russia for vetoing a United Nations Security Resolution which would have increased pressure on President Assad's rule and could have paved the way for military action.
Col Kemp 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.
"Aside from the stance of Russia and China, the absence of a coherent opposition movement that could replace Assad without potentially increasing bloodshed will be high among those reservations.
"Up to the point of Assad's collapse, we are most likely to see a continuation or intensification of the under-the-radar options of financial support, arming and advising the rebels, clandestine operations and perhaps cyber warfare from the West.
"After any collapse, however, the military options will be seen in a different light."
A spokeswoman at the Foreign Office said the Government was committed to a peace plan drawn up by former United Nations secretary-general Kofi Annan.
Foreign Secretary William Hague has previously called on Russia to increase pressure on the Syrian regime to implement the plan, while admitting "cautionary words about military intervention" were understood by ministers.
A Foreign Office spokeswoman added the UK Government was seeking a peaceful resolution to the crisis without the need for military intervention.
She said last week's UN Security Council Resolution was tabled under Article 41 of the UN Charter, and related to sanctions against Syria and not military action.
The legally binding resolution would compel Assad to comply with Annan's plan, she added.