By Paul Reynolds
World Affairs correspondent, BBC News website
After a shaky start when the concept was first floated at the G8 summit by Tony Blair and Kofi Annan, the idea of an international force in southern Lebanon has taken hold.
There are nearly 2,000 Unifil soldiers in the area
The US Secretary of State Condoleezza Rice said after the Rome meeting on Lebanon: "What we agreed upon is there should be an international
force under UN mandate that will have a strong and robust
capability to help bring about peace and help humanitarian
However, a number of conditions will have to be fulfilled if it is to be effective.
For a start, it has to have a mandate. And there would have to be a ceasefire. Its size and composition have to be agreed.
The Israeli attack on the UN post in south Lebanon which killed four UN observers will make potential contributing states even more keen on having an agreed framework before they accept deployment.
One proposal envisages the force being temporary in nature - to hold the ground while or after Hezbollah is withdrawn from the border and the authority of the Lebanese government is asserted by the deployment of the Lebanese army.
Dr Rice has indicated that the force could be deployed along the border for two or three months, and then expand north to help the Lebanese army.
The Israeli Defence Minister Amir Peretz has said that Israel will establish a "buffer zone" along the border until an international force arrives.
The Israeli Prime Minister Ehud Olmert is reported to have told a Knesset committee that the zone would be about 2km deep, rather smaller than Mr Peretz had indicated.
The eventual aim would be to implement UN Security Council resolution 1559, passed in September 2004, which called for the disbanding of the Hezbollah and other militias and the re-assertion of government control.
For that to happen though, another condition would have to be fulfilled. Hezbollah would have to agree (with the Lebanese government, of which it is actually a part) on a withdrawal.
The risk in all this is that the buffer zone would be set up and nothing else.
The force's powers would have to be internationally agreed and enshrined in a new Security Council resolution. That resolution would also call for an Israeli withdrawal.
It will be tricky to agree on a force. If it is too strong it could be seen by Hezbollah as an enemy.
Timor Goksel, a former spokesman for the existing UN force there, Unifil, is still in Lebanon and his view on the proposed force is clear.
"If it was seen as a force that would take action against Hezbollah it would be perceived as an occupation force," he told the BBC.
If it is too weak it will not satisfy Israel. Israel is determined not go back to the precarious position that existed before the violence started.
And a key issue is who would send troops. Nobody has stepped forward publicly so far.
The Lebanese government has mentioned EU countries. Israel and the US seem to prefer Nato, which has the Americans in a central role.
And how big would a force be?
Unifil has nearly 2,000 troops already in place, from China, France, Ghana, India, Ireland, Italy, Poland and Ukraine.
Some of these could be used. The aim will be to get heavy hitters. Turkey is a possibility. So is Germany, though the Germans have stressed the importance of getting a ceasefire first. But the US will not take part, for obvious political reasons.
The US lost 241 marines and other troops in a suicide attack in Beirut in 1983 (Hezbollah was blamed). France also suffered from that attack.
The UK would not participate, if only because of overstretch elsewhere.
The timing of deployment is important.
However, British Foreign Secretary Margaret Beckett has said a ceasefire should not be linked to the "putting in place of a full force". The implication is that it could go in stages.
The history of Unifil is perhaps instructive for present policymakers.
For we have been here before.
In 1978, Israel attacked into southern Lebanon up to the Litani River some 20km to the north.
That offensive was to drive out the Palestinians. Palestinians had just carried out a massacre of Israeli civilians in a bus on the coastal road.
Very quickly there followed Security Council resolution 425, which called on Israel to withdraw (it took 18 years to do so).
It also set up, for an initial six-month period, the "United Nations Interim Force in Lebanon" (Unifil). The word "interim" has been stretched to breaking point.
Unifil was supposed to confirm the withdrawal of Israeli forces, help "restore international peace and security" and assist the return of the "effective authority" of the Lebanese government.
Aims not carried out
It did none of the above. Instead, Israel stayed and set up its own proxy militia, mainly Christian, known grandly as the South Lebanon Army.
The authority of the Lebanese government was not restored. Unifil had no authority. There was no proper peace and security.
Timor Goksel, the genial Turk who was the spokesman for many years, used to organise visits into the Unifil area for foreign journalists.
The Irish battalion was the destination of choice for many western reporters.
A sergeant with a sub-machine gun under the passenger seat would drive reporters around, the battalion offered a decent lunch and a lot of information about what was going on, but like the other contingents could only observe and report. Unifil was diminished into a monitoring group.
In 1982, Israel invaded up to Beirut; there were further major operations in 1993 and 1996.
In 2000, Israel, now harried by Hezbollah, the Lebanese Shia "Party of God", decided that it had had enough and withdrew.
Hezbollah moved in. The Lebanese government did not. Hezbollah accepted the presence of Unifil only because Unifil did not hinder Hezbollah.
This time, Israel has gone in again. And this time there is also talk of an international force and of the authority of the Lebanese government being restored and its army deployed to the border.
Maybe this time it will happen, but in the Middle East, history can repeat itself and each time as tragedy.