The Serbs control about 70 per cent of Bosnia, mostly in the north and east. But this picture is deceptive. In order to hold their position in and around Banja Luka, their Bosnian headquarters, the Serbs need to keep open a thin corridor of territory to the east, near the town of Brcko. This is the route by which Serbia has sent supplies of munitions, fuel and food by helicopter and truck to the Banja Luka Serbs.
If this route is cut off, the Serbian position will look very vulnerable. Already Bosnian government troops have taken control of the southern half of Brcko. Strict Western enforcement of a 'no-fly zone' in the area would sever the air lifeline between Serbia and the Serbs of northern Bosnia.
Sarajevo radio said yesterday that, in an apparent attempt to widen the supply corridor, the Bosnian Serbs had started to send tanks and other equipment from Banja Luka to Gradacac, 20 miles west of Brcko.
Mark Mazower, a Balkan expert at Sussex University, believes that Western powers could take other steps without serious additional risks. 'One could lift the arms embargo on the Bosnian government forces and unobtrusively arm them so that they could enforce the no-fly zone themselves with ground-to-air missiles, and shoot down the helicopters. The evidence around Brcko is that the Bosnian government forces could cut the supply corridor,' he said.
Mr Major went to Washington to explain to President Bush British doubts over such proposals, and to point out that the West must be careful not to endanger the lives of UN peace-keepers and humanitarian aid workers in Bosnia. The Bosnian Serb leader, Radovan Karadzic, indicated last week that Western military enforcement of the air exclusion zone would, in his view, make them legitimate military targets. Mr Major faces no easy task, as the US has come round to the view that action against the Bosnian Serbs is necessary, even if it means a temporary halt to humanitarian relief work.
The Serbs' ultimate intention is to merge Bosnian areas under their control with Serbia and with Serb-held lands in western and eastern Croatia. In this way, Serbian politicians and generals hope to realise their ambition of an enlarged Serbian state.
In pursuit of this goal, the Serbs are willing to let the Croats annex western Herzegovina with its largely Croatian population. The losers in this scenario are, of course, the Bosnian Muslims. Before the war started, they were the republic's largest nationality, with 44 per cent of the 4.4 million people, but if a Serb-Croat carve-up took place, they would be left with, at best, a truncated state.
The West's more aggressive policy is aimed partly at appeasing Islamic countries, which are so outraged at the fate of Bosnia's Muslims that they are threatening to offer them armed support from 15 January. In Turkey, there is particularly strong pressure for action from the armed forces and parliament.
The West is alarmed that Islamic intervention would not only deepen instability in the Balkans but also spread tensions farther afield, by playing into the hands of conservatives who are gaining the ascendancy in Russia's defence ministry and foreign ministry. In the same way that Mikhail Gorbachev's enemies attacked him for supporting the West against Iraq, a close Soviet ally, so Boris Yeltsin's opponents dislike his policy of siding with Western countries against Serbia, a traditional Russian friend.
Leading article, page 20