To Sarajevo, by way of Riyadh: What do Saudi princes, Afghan guerrillas, Iranian ammunition and the United States marines have to do with Bosnia? Just wait and see, says Robert Fisk

A group of American officials turned up at the Croatian port of Ploce, a few miles from the Bosnian border, earlier this month. They wanted to talk to the harbour master. They wanted to know how many ships could be berthed at the Ploce quays in any one day. How many ships could be turned round in a day? How big were the railyards? It did not take the harbour authorities long to guess why their unexpected visitors had arrived here, within sight of the Bosnian mountains. If the marines are going to be sent into the Balkans, Ploce will be their port of disembarkation, the road from the railyards at Metkovic their first line of advance towards Sarajevo.

John Major may be fearful of British casualties in Bosnia; his Defence Secretary - using Wehrmacht statistics to dismiss any idea of an offensive against Serbian forces - may remind one of the timidity of Thirties appeasers; but already the UN role in the Balkans is undergoing a series of subtle changes in preparation for possible military action. At least two US officers are now part of the new UN headquarters outside Sarajevo, where the largest armoured force is no longer Serbian but British. The so-called 'Bosnia- Herzegovina Command' of the international army at Kisiljak includes US, British, Danish, French, Norwegian and Canadian officers in what looks suspiciously like a miniature version of the old Nato Northern Command.

It is, in effect, a rival UN force in the Balkans. Officially, it is part of the same, discredited UN protection force that has performed so impotently over the past 18 months. But in Kisiljak the national flags are larger than the emblems normally displayed by UN personnel; the Union Jack is larger, the UN flag smaller than usual. British officers can even be heard talking of the British protection force rather than the UN protection force.

Nor is this surprising. British UN officers have quietly instituted a military logistics 'tail' far larger than necessary for the 2,400 British troops in Bosnia; they could now cope with the immediate arrival of a further 5,000 British soldiers, although artillery and helicopter support would have to be provided for any offensive action. Spanish legion troops have now secured the main highway from the coast through Mostar towards Sarajevo. Is it really any surprise that the US marines in Mogadishu have been telling reporters that they all expect to move on to Bosnia?

Somalia is more than just a test case. Military involvement of any kind - however ostentatiously peaceful its intentions - bestows influence as well as power upon the foreign nations committing troops to the operation. Thus feeding the hungry in Somalia has given the United States military bases adjacent to Kenya, whose election crisis is growing daily more serious, and scarcely 500 miles from Sudan, whose Islamic leadership is already being demonised by the West.

Saudi Arabia, Washington's policeman in the Gulf, has moved its own troops into Somalia - one of their senior officials in Mogadishu is a close adviser to Prince Bandar, the Saudi ambassador to Washington - while at the same time demanding Western action in Bosnia.

Amid the fears and sensitivities expressed by Western statesmen over the Balkans, little attention has been paid to Saudi Arabia's role in Bosnia. Saudi television is now broadcasting 24-hour-a-day news reports of the horrors endured by Bosnia's Muslims. Almost dollars 50m worth of aid for refugees has come from King Fahd alone. And it was the Saudis who hosted this month's Islamic conference in Jeddah that demanded Western action to save the Moslems of Bosnia. If no such help was forthcoming, the conference decided, then Saudi Arabia and its Islamic allies would send weapons to the Bosnians.

No one noticed the connection between this warning and the UN's subsequent threat of a 'no-fly' zone over Bosnia, to come into effect in mid-January - the very date the Saudis had set in their ultimatum. For it would be ridiculous to suppose that the Saudis did not act in concert with their American allies.

All across the Muslim states of the former Soviet Union, in Afghanistan and now in Somalia, too, Saudi largesse is being dispensed in an attempt to diminish Iranian influence. Riyadh and Tehran are fighting it out with the help of their proxy militias in the streets of Kabul. And the Saudis, with America's support, are now determined to prevent the Muslim refugees of Bosnia falling into Iran's hands.

Iranian aid and guns have certainly been arriving in the former Yugoslavia. Iranian ammunition was carried by Bosnian Muslim guerrillas who penetrated Serbian Krajina in September. In Bosnia itself, 147 members of the pro-Iranian Hizbollah militia from Beirut have now arrived to provide arms and guerrilla training to Muslim fighters. Several dozen militiamen from the northern Lebanese city of Tripoli have travelled to Bosnia for the same reason. Algerian, Kuwaiti and Saudi fighters have turned up near Travnik and - although not in the huge numbers that the Serbs claim - have re- established some important links with their Bosnian Muslim hosts.

Dozens of Bosnian Muslims, it transpires, fought with the mujahedin in Afghanistan. Their comrades were those same Algerians and Kuwaitis who have now come to Bosnia. 'Afghanistan was our school,' one Bosnian Muslim officer told me in Travnik. 'Instead of fighting the Communist Russians outside Kabul, we are now fighting the Communist Chetniks (Serbs) outside Travnik. Even their tanks are the same. In Afghanistan, we hit the T-55s in the mountains. Here, we hit T-55s in the mountains.'

Thus has the experience of Afghanistan brought together a small but dedicated army of Muslims in Bosnia. And as the Balkan war continues - and perhaps spreads to include Kosovo and Albania - so Saudi Arabia will try to extend its control over the fighters. Islamic resistance in the former Yugoslavia is one thing, Islamic revolution quite another. It will be of no comfort to the Saudis to know that one of their citizens has just been blowing up Serbian tanks near Tarcin with a new shoulder-fired missile launcher; another veteran of Afghanistan, he opposes the monarchy in Riyadh on the grounds that they are infidels.

Rescuing the Muslims of Bosnia is therefore a political as well as a humanitarian task for Saudi Arabia, and one that may have to be accomplished with Western fire-power, just as Kuwait had to be liberated by Western tanks and aircraft. The equation is familiar: allied ground troops and American air cover. Saudi funds could be used to support such a mission, just as they were in the Gulf. While theoretically undertaken by the UN protection force, the operation would belong to Nato troops, undertaken with the intention of setting up a UN protectorate in Bosnia.

It would be impossible to claim afterwards that this had been a 'clean' war. While allied troops were confronting the Serbs, Serbia's Croatian enemies would be using such an offensive to perform their own acts of ethnic cleansing against both Serbs and probably Muslims, too. Even when the first British troops were arriving in Bosnia this autumn, the Croatians burned the Muslim inhabitants out of Prozor, a disgrace that an outnumbered British advance party witnessed from a distance but could do nothing to prevent. If Croatian militiamen massacred Serbian villagers, the West would be held to blame.

Yet it is important to remember that the West escaped censure when more than a hundred Palestinians were tortured and slaughtered in liberated Kuwait and when several hundred thousand Palestinians were expelled from the country - an act of ethnic cleansing that did not trouble proponents of the 'New World Order'.

There would be other costs to an American-led war in the Balkans. The European Community would have to reassess its aspirations. If America had to bail Europe out in Bosnia, Europe would be in no position to challenge American influence on this side of the Atlantic. There could be no more rows over Gatt, and few claims from the ancient countries of Europe that they do not need America's moral or military strength. America's supremacy in Europe would be unchallenged; Washington would hold more power in the Continent than ever before.

But if Europe cannot control the Balkans, then America will have to do so. Without American ground troops, the UN has proved to be a disaster in what used to be Yugoslavia. The EC observers - most of them military intelligence officers - have little influence over the horrors of Bosnia. The EC itself has managed to push Macedonia towards war because one of its members, Greece, refuses to accept the country's name. The probable victory of Slobodan Milosevic over Milan Panic will remove the last remaining hope of Serbian moderation. Repeatedly over the past six months, European leaders have sounded like the statesmen of the mid-Thirties, insisting that there must be no more war - not because they have the will to prevent it but because they lack the courage to do so.

Which is why America will come to the Balkans. Meanwhile, watch the Saudis; observe the activities of the so-called Bosnia-Herzegovina Command of the United Nations; remember Somalia. We may not have long to wait.

Andrew Marr is on holiday.