More Nato troops have been sent to Kosovo to stem communal clashes this week.
Nato's bombing campaign of 1999 has been held up as a successful humanitarian intervention.
The KLA fought for several years for Kosovo's independence
But the renewed unrest raises more awkward questions about the value of military force as a response to conflicts and crises.
The federal state of Yugoslavia, formed after World War II, held many different nations and regions within its borders.
Legally, Kosovo is still a province of Serbia - then the biggest of six Yugoslav republics.
But Serbs were only a small minority of the Kosovo population.
By the time of the last census, in 1991, about four-fifths of its people were ethnic Albanians.
Rise of nationalism
By then, ethnic tensions had caused the break-up of Yugoslavia.
Kosovo Serbs had been complaining for years of persecution at the hands of their Albanian neighbours.
The situation gave rise to Serb nationalism - Slobodan Milosevic was elected president of Serbia, promising to protect them.
The substantial autonomy Kosovo had enjoyed was suspended to be replaced by a virtual police state, imposed from Belgrade.
The subsequent years of Serbian repression led to ethnic Albanian demands for independence - later to the formation of the Kosovo Liberation Army, KLA, to fight for it.
KOSOVO: KEY DATES
24 Sept 1998: Nato issues ultimatum to Milosevic to stop crackdown on Kosovo Albanians
24 Mar 1999: Nato begins air strikes against Yugoslavia over Kosovo
10 June 1999: Air strikes suspended after Milosevic agrees to withdraw troops. UN approves peace plan for Kosovo, establishes K-for peace force
11 June 1999: Nato troops enter Kosovo
10 Dec 2003: UN unveils road map on conditions Kosovo must meet by mid-2005 for talks on final status
17 Mar 2004: Serbs and Albanians clash in the worst violence seen since 1999
A guerrilla attack on a Bosnian Serb refugee camp in Kosovo in 1996, and a subsequent campaign of harassment, brought savage reprisals against ethnic Albanian civilians by the Yugoslav army.
Enter the international community. In 1998, the Organisation for Security Co-operation in Europe sent a Kosovo Verification Mission to oversee a ceasefire.
But, while the army withdrew, the mission did nothing to stop the KLA from taking over its positions and stepping up its guerrilla war.
A subsequent BBC investigation established that most truce violations were coming from the Albanian side.
A political blueprint for the province was presented at the Rambouillet summit the following year - by which time the Yugoslav army had been sent back in.
Initially, the KLA was reluctant to sign the deal because it did not stipulate independence for Kosovo.
So Madeleine Albright, then US secretary of state, intervened to promise a possible referendum on a final status - and two weeks later, the Kosovo Albanians signed.
The Yugoslav delegation could not agree to that - or provisions giving Nato unimpeded access, including in Serbia proper - and the alliance began its 78-day campaign of bombing.
It ended with a resolution of the UN Security Council, which dropped the clause about the referendum, saying merely that the Rambouillet text would be "taken fully into account".
This was necessary to get the Serbs to agree to pull their forces out of Kosovo, and to avoid a Russian veto at the UN, but it is responsible for the continuing blight and uncertainty over the province's political future.
Face-to-face talks between Serbs and Kosovo Albanians began late last year, but the legacy from the violence of the late 1990s is one of mutual mistrust.
The stated aim of Nato's intervention was to "avert" a humanitarian catastrophe.
But it triggered an exodus of ethnic Albanian refugees being expelled by the Serbian security forces, which the alliance then promised to "reverse".
The Albanians were able to go home, but about 200,000 of Kosovo's ethnic minorities left to be refugees in Serbia or displaced within Kosovo.
It is one factor in the growing power of nationalist politics in Serbia.
Critics of Nato's intervention say it was one-sided; the violence left an embittered population and a number of unresolved, and potentially dangerous problems.
The trouble of recent days comes amid an intensifying debate on the merits - or otherwise - of military force as a response to conflicts and crises.