The British insist peace-keeping and humanitarian operations, which depend on the consent of the people of the country concerned, are different from "peace enforcement", which involves taking sides and using considerable firepower. Once the divide of "consent" between peace-keeping and peace enforcement is crossed, it is impossible to return. If a force on a peace- keeping mission cannot carry it out, it should withdraw, a view emphasised by the former UN commander in Bosnia, Lieutenant-General Sir Michael Rose.
The US view, until recently, was that peace-keeping and peace enforcement were distinguished only by different levels of violence. US experience in Somalia showed this does not work. Once the dividing line is crossed, further peace-keeping attempts are, as a UN official said, "like giving first-aid to a wounded rattlesnake".
British Army teams, drawing on their experience in Bosnia, have won over the US. The new US Army manual, Peace Operations, is much closer to British views. The British manual ranges from support of humanitarian aid in a benign environment, through "wider peace-keeping" in a hostile environment - "blue-helmet" operations such as Bosnia - up to "peace enforcement".
But there is a sharp divide between wider peace-keeping and peace enforcement. Gen Rose called it the "Mogadishu Line", a reference to Somalia, where the US changed the aim of the mission from peace-keeping to trying to destroy the warlord Mohamed Farah Aideed.
The new US manual concurs: "US policy distinguishes between peace-keeping and peace enforcement. Both are peace operations. However, they are not part of a continuum allowing a unit to move freely from one [objective] to the other."