Before the battle:

In 1354, Islam invaded Europe. In that year, the armies of the Ottoman Turks moved west and conquered the Aegean port city of Gallipoli. By 1389, much of the Balkans (Bulgaria and Serbia) were vassal states of the Ottoman Empire (see the map at right from The Ottoman Empire: 1326-1699, Stephen Turnbull, Osprey, 2003).

The first counterattack was launched in 1363. Serbs, Bosnians, and Wallachians joined a Hungarian army (under the Hungarian king, Louis the Great) and marched to Edirne (formerly Adrianople). There they planned to attack Sultan Murad and the Turks. Disaster struck when the Christian force was ambushed by Turkish cavalry at the river Maritza. The Christians were routed and fall back in defeat. 

Again, in 1371, the Serbs attacked the Turks, and again the Serbs were defeated. This second defeat came on 26 September 1371, at the battle of Cernomen. All the Serbian leaders were killed. This battle, also known as the second battle of the Maritza, is known in the Turkish chronicles as "Sirf sindigi" (the destruction of the Serbs).

The Turks could not be thrown back. In 1385, they captured Sofia and, the next year occupied the Serbian town of Nis.

In 1389, a Serbian leader, Lazar, raised an army to drive the Turks from the Balkans. Lazar raised contingents from Serbia (included forces led by his son-in-law, Vuk Brankovic), Bosnia, Wallachia, Albania, and Hungary.

This Christian force met the Turks at Kosovo Field (Kosovo Polje), the Field of Blackbirds, on St. Vitus's Day, 1389.

The battle and its aftermath:

The Kosovo battle resulted in heavy losses on both sides, but was a devastating loss for the Serbs. Most of the Christian leaders and nobility were killed or driven into exile. Sultan Murad was assassinated behind his lines by a Serbian knight, Milos Obilic, and Lazar was captured and beheaded by the Turks.

Serbian epic songs give two contradictory reasons for the Serbian defeat: the treachery of Vuk Brankovic-- which seems to have no basis in fact--and Lazar's decision before the battle to sacrifice his earthly kingdom for a heavenly kingdom, to lead his men into battle knowing what the tragic outcome was to be.

Complete Turkish domination of Serbia was achieved very slowly by Sultan Murad's successors. A second battle of Kosovo was fought in 1448. The final and conclusive battle was not fought until 1439 for the fortress at Smederevo on the Danube, Nonetheless, it is Kosovo which has lived in the Serbian tradition as the moment of annihilation and enslavement.

Sources:

There are no detailed historical accounts in English of the battle of Kosovo. The basics can be found in general histories such as The Crusades by Jonathan Riley-Smith and The Ottoman Empire: 1326-1699.

The best accounts are the cycle of epic poems created by the Serbian people. Originally collected by Vuk Stefanović Karadžić in the 19th-century, the poems can be found in The Battle of Kosovo translated from the Serbian by John Mattias and Vladeta Vuckovic. One of the most famous of these poems, The Downfall of the Kingdom of Serbia, is given in its entirety below.

The Downfall of the Kingdom of Serbia

(translated from the Serbian by John Mattias and Vladeta Vuckovic)

Yes, and from Jerusalem, O from that holy place,
A great gray bird, a taloned falcon flew!
And in his beak he held a gentle swallow.
But wait! it's not a falcon, this gray bird,
It is a saint, Holy Saint Eliyah:
And he bears with him no gentle swallow
But a letter from the Blessed Mother.
He brings it to the Tsar at Kosovo
And places it upon his trembling knees.
And thus the letter itself speaks to the Tsar:
"Lazar! Lazar! Tsar of noble family,
Which kingdom is it that you long for most?
Will you choose a heavenly crown today?
Or will you choose an earthly crown?
If you choose the earth then saddle horses,
Tighten girths- have your knights put on
Their swords and make a dawn attack against
The Turks: your enemy will be destroyed.
But if you choose the skies then build a church-
O, not of stone but out of silk and velvet-
Gather up your forces take the bread and wine,
For all shall perish, perish utterly,
And you, O Tsar, shall perish with them."

And when the Tsar has heard those holy words
He meditates, thinks every kind of thought:
"O, Dearest God, what shall I do, and how?
Shall I choose the earth? Shall I choose
The skies? And if I choose the kingdom,
If I choose an earthly kingdom now,
Earthly kingdoms are such passing things-
A heavenly kingdom, raging in the dark, endures eternally."
And Lazarus chose heaven, not the earth,
And tailored there a church at Kosovo-
O not of stone but out of silk and velvet-
And he summoned there the Patriarch of Serbia,
Summoned there the lordly twelve high bishops:
And he gathered up his forces, had them
Take with him the saving bread and wine.
As soon as Lazarus has given out
His orders, then across the level plain
Of Kosovo pour all the Turks.