Because they are terrified. In the past four months Rwanda has witnessed massacres on an appalling scale, mainly of the minority Tutsi people by the majority Hutu. Now a rebel army composed almost entirely of Tutsis has taken over, and Hutus fear they will be massacred in turn.
That is not the whole story. Some of those now escaping Rwanda are members of the mainly Hutu army of the defeated government or of Hutu murder gangs, and there is concern that they plan to regroup in exile and launch an invasion. In addition, some refugees are Tutsis who fled weeks ago at the height of the worst massacres. They now fear death at the hands of Hutu refugees. The situation is bewildering and tragic, the latest page in a catalogue of horrors for which it is hard to find precedents.
How did this crisis begin?
A war has been smouldering away in Rwanda since 1990. The spark that set off the conflagration was the death in April of Juvenal Habyarimana, the Hutu strongman who had ruled the country for more than 20 years. He was killed when the plane in which he was travelling with Cyprien Ntaryamira, the President of neighbouring Burundi, was brought down by a missile. No one knows who was responsible, although Hutus immediately blamed Tutsi dissidents.
Immediately after the assassination Hutu militias - gangs of killers raised, trained and armed by the government - set upon the Tutsi minority. This was a genocidal campaign planned, prepared and executed with chilling efficiency. The bloodshed was appalling, even by the cruel standards of modern Africa. Pregnant women had babies cut out of their bellies, rivers were choked with corpses, entire villages were wiped out.
The Hutu killers systematically combed the countryside for every Tutsi they could find, as well as any Hutu suspected of sheltering them or supporting reconciliation with them. In the space of just a few weeks at least 500,000 people are thought to have been killed - some estimates put the figure nearer a million. Hundreds of thousands are unaccounted for.
What sort of country is Rwanda?
It is Africa's most densely populated country. With an area of about 10,000 square miles, it is a little larger than Wales, but (at least before the current catastrophe) it had more than twice the population, or 7-8 million.
Rugged countryside, great daily changes in temperature, and only limited rainfall make it a poor place for agriculture. The main cash crops are coffee and tea, and tin is another export. Only 1 per cent of the labour force is engaged in manufacturing. The low world coffee price of recent years pushed Rwanda heavily into debt.
Politically, Rwanda is no arbitrary colonial creation. Like its neighbour Burundi, it was a kingdom from about the 15th century and settled into roughly its present boundaries in the 19th century, before the arrival of the first white men. The Hutu and Tutsi peoples who made up the population of the two countries are not found in any numbers elsewhere.
During the 1890s Rwanda and Burundi became part of German East Africa, but they were occupied by the Belgians during the First World War and subsequently placed under Belgian control, as 'Ruanda- Urundi', by League of Nations mandate. Both countries became independent in 1962.
What is the difference between Hutus and Tutsis?
There is no simple, satisfactory answer. When the European colonists arrived, they found a well-estabished social hierarchy in which the Tutsis (who account for just 15 per cent of the population) were the cattle-owning aristocracy and the Hutu lowlier peasant farmers.
Whether 'Tutsi' and 'Hutu' were social classifications or racial labels, however, is not clear. The two peoples shared language (Kinyarwanda), culture and territory.
The Belgians chose to exploit the social distinction, and the effect was to accentuate it. Where education was provided, it was given to Tutsis, who to this day dominate the professional classes. Tutsis also became more Westernised and, for example, tended to have smaller families.
It is said that the two differ in apearance: the typical Tutsi is taller, finer-featured and lighter-skinned than the Hutu. But again it is not known whether the explanation for this is racial, or the result of social advantage and diet. In any case, many on both sides do not fit these descriptions, and intermarriage has blurred many of the distinctions. This blurring continues into politics and war: Hutus have killed Hutus and Tutsis have killed Tutsis in the current fighting.
As a rule of thumb, however, it could be said that a Tutsi is someone from a family which is or was part of the traditional social elite. They may now be poor peasants and they may even oppose Tutsi rule, but they are still Tutsis.
Can they not live peacefully together? On the evidence of the post-colonial experience, no. Since independence in 1962, Rwanda and Burundi have followed different paths, both marked by cruelty and injustice.
In Burundi, the Tutsi ruling class clung to power by brutally suppressing the Hutu majority at any hint of unrest. This worked for more than 30 years, but then last year everything changed. The country's leader, Pierre Buyoya, called elections he felt would achieve reconciliation. He was defeated and a Hutu, Melchior Ndadaye, became president.
When Ndadaye sought to redress some of the imbalances, within the country, notably over land ownership and in the army, the Tutsi elite felt threatened. Ndadaye was killed by Tutsis; the Hutus massacred thousands of Tutsis and the Tutsi army in turn slaughtered thousands of Hutus.
In Rwanda it was the majority Hutus, fed up with being treated like serfs, who seized power after independence. The leaders of the Tutsi community fled to Uganda, where they set up a rebel movement. Ever since, attempted invasions or fears of invasion have served to feed Hutu-Tutsi tensions.
Tutsis were excluded from the army and government jobs; they tended to be blamed for the country's problems and there were occasional bouts of violence.
This has been exacerbated by pressure on the land. The population of Rwanda is growing extremely rapidly: it was 5 million in 1978 and it is now more than 7 million.
This sets Hutu against Tutsi because tending herds (as Tutsis still do) requires far more land than does growing vegetables. Each side feels its ability to feed itself is being threatened by the other.
Just who are the 'rebels'?
They are the political and military successors of the dispossessed Tutsis who fled to Uganda 35 years ago. Down the years these exiles were persecuted by successive Ugandan regimes until eventually they found allies in a group called the National Resistance Army, led by Yoweri Museveni. In the early 1980s, many young Tutsis joined the NRA. They gained a reputation for leadership and discipline and helped to bring Museveni to power in Uganda in 1986.
After this they began to plot their own return from exile and in 1990 about 2,000 of them took their guns and crossed over into Rwanda. They quickly made gains against the less experienced and less disciplined government troops, and before long they controlled a large enclave of northern Rwanda.
Last year President Habyarimana reached a peace agreement with the rebels - the Arusha settlement - which entailed a dilution of Hutu control of the country. Whether he intended honouring this we will never know, as he was killed in April.
Since then, the rebels have seized virtually the whole country and installed themselves as the government in the capital, Kigali.
Their organisation is called the Rwanda Patriotic Front (RPF) and it has a Hutu chairman (Pasteur Bizimungu, now declared president), but it can hardly be described as broad- based. Its soldiers are 90 per cent Tutsi and most Hutus regard it as an exclusively Tutsi organisation. Because most of the RPF were born and raised in Uganda, their European language is English rather than French. This serves to emphasise differences.
Who has provided the weapons for this war?
The RPF has obtained almost all its arms from or through Uganda. Uganda was awash with weapons from the Amin and Obote eras. It has been possible to obtain a Kalashnikov rifle there in exchange for a chicken. The good relations between the RPF and Uganda's ruling NRA ensured continuous supplies for the rebels during their campaign.
The now-deposed government of Rwanda had its army trained and armed almost exclusively by France, which remains a power in Africa and has long been keen to draw this former Belgian colony into its sphere of influence.
What has the UN been doing about it?
UN troops were first sent to Rwanda to monitor the peace process set out in the Arusha settlement which was struck last year. They were unable, however, to prevent the rapid escalation of the conflict last April, indeed in the first orgy of bloodletting after the asssassination of President Habyarimana, 11 Belgian UN soldiers were killed. The 80- man UN contingent which was stationed along the Rwanda- Uganda border to ensure that Uganda was not supplying arms to the RPF also clearly failed to fulfil its mission.
As the scale of the massacres became evident and the first refugees spilled across Rwanda's borders, the world was slow to react. Caution learned in Somalia and Bosnia held most governments back: they feared becoming embroiled; they could not see what sort of operation would do good; their forces were already overstretched by UN peacekeeping work; they were worried about the cost of an operation in such a remote place.
The US was particularly reluctant to become involved, but in mid-May, five weeks after the killings began, it dropped its objections and the UN Security Council finally agreed to despatch a force. They would be allowed to protect themselves and civilians and to defend aid supply routes, but they would not use force to stop the fighting.
This force, however, has been difficult to assemble. A total of 2,000 UN troops from Ghana, Zimbabwe, Niger and Canada are now promised for Rwanda by the end of August.
What have the French been doing?
On 19 June, President Francois Mitterrand announced that French paratroops were being deployed to Rwanda from their nearby African bases as an interim measure while the UN force was mustering.
Although it had UN backing, this move was widely criticised. France, it was said, was shoring up its influence in Africa. Moreover, as long-time allies of the Hutu government, the French would inevitably be seen as partial. In reply, France accused the international community of 'culpable inaction'.
Using 2,500 troops, the French declared a 'safety zone' in the south-west of Rwanda where, by their account, protection was given to 1.5 million people.
Soon, however, France had to acknowledge the victory of the RPF over its former ally, and then hundreds of thousands of Hutus began spilling out of the safety zone into neighbouring Zaire.
France has declared its operation a success and plans to withdraw its troops by 21 August, when the UN force should be arriving. On balance, they appear to have done some good, but there are suggestions that, by accident or design, the French have given the Hutu forces an opportunity to regroup.
Why were the relief agencies taken by surprise? The disaster was predictable, but not the scale. There were plenty of depressing precedents for the Tutsi- Hutu bloodletting - massacres in Burundi in 1965, 1972, 1988, 1991 and 1993 and massacres in Rwanda in 1959, 1961 and 1963. But this is far and away the bloodiest chapter in Rwandese history.
One estimate suggests two- thirds of the population have fled their homes. Half or more of the Tutsi population may be dead. As many as 2 million of a total Rwandese population of less than 8 million have fled across the borders into neighbouring Burundi, Tanzania, Uganda, and Zaire.
This is more than anyone could expect the international aid organisations to be prepared to cope with.
As Sylvana Foa, the spokeswoman for the UN High Commissioner for Refugees, said: 'Our workers are facing a human wall of people, a real tidal wave just advancing towards them.'
Recent comparisons are: in three months, 2.2 million Rwandese have been displaced. In Kurdistan in 1991, during the equivalent period, 1.4 million refugees took to the roads; in former Yugoslavia there have been 4 million refugees, but over three years; and in Afghanistan, 6 million refugees over 14 years. The UNHCR reckons the number of Rwandans it needs to clothe, shelter and feed will eventually exceed 4 million.
Can anything be done to avert catastrophe?
It is too late to prevent a great human tragedy. Most agree that the urgent need is for the refugees and displaced people to return home as soon as possible. Most have left their crops in the fields, only weeks before harvest. Baroness Chalker, Britain's Minister for Overseas Development, said: 'It's vital that refugees return to Rwanda, where aid supplies can reach them. They can begin harvesting their crops to feed themselves.'
But people are too frightened to go home, and as long as that fear remains, there will be a humanitarian crisis so grave that all aid agencies agree that they cannot cope. They have called for substantial military help to airlift and distribute supplies of food, medicines, water, and vehicles. This is what President Bill Clinton set in motion at the end of last week.
Will the new Rwandese government be better than the old?
It is possible. The RPF is relatively disciplined, and has shown remarkable restraint in dealing with the Hutus who slaughtered their Tutsi fellow countrymen, and this offers some hope that Hutu refugees can be reassured.
In the longer term there must be grave doubts. Neither in Rwanda in the past nor in Burundi at present have Tutsis ever shown any inclination to share power with the Hutus. Rwanda is likely to exchange one-party majority rule for one-party minority rule.
And given the the bitterness and hatred created in the past three months, on top of decades of mutual resentment, it is hard to see a spirit of reconciliation taking root. On both sides, moreover, there are wild men ready for more killing. The newer recruits to the RPF, many of whom have lost close relatives in the Hutu massacres, may not prove as disciplined as the Uganda veterans.
On the Hutu side, the brutal militias continue to murder Tutsis even now, in the refugee camps, while the defeated government talks of rallying its army in Zaire, ready to renew the war with the RPF.
Could the war spread to other countries?
In a way it already has, since there is violence among the refugees, but the nightmare now is that something on a similar scale could happen in Burundi, where the Tutsi rulers continue to keep the lid on a Hutu majority, and the tradition of massacre is just as strong.