The Battle of Covadonga was the first major victory by a Christian military force in Spain following the Islamic Moors' conquest of that country in 711. Taking place about a decade later, most likely in the summer of 722, the victory at Covadonga assured the survival of a Christian stronghold in northern Spain, and today is regarded as the beginning of the Reconquista.
Seven years after the Islamic conquest of Spain, Pelayo, a nobleman of the country's former rulers, the Visigoths, managed to expel a provincial governor named Munuza from the district of Asturias in the northwestern part of the Iberian peninsula. He held the territory against a number of attempts to re-establish Islamic control, and soon established the Kingdom of Asturias, which became a Christian stronghold against further Islamic expansion. Pelayo was unable to keep the Muslims out in many cases, but neither could they defeat him, and as soon as they would leave Asturias, he would re-establish control.
Pelayo didn't attempt to force the issue, and it was a Moorish defeat elsewhere that probably set the stage for Covadonga. On July 9, 721, an Islamic force that crossed the Pyrenees and invaded the Kingdom of the Franks was defeated by them near Toulouse, in modern France. The Moors returned to Spain and decided to consolidate their holdings there before taking on the Franks again. One of the key tasks in achieving that would be to destroy Pelayo's rebel stronghold.
In the late summer of 721, a Moorish general named Alqama led his men into Pelayo's territory and overran much of it, forcing Pelayo to retreat deep into the mountains of Asturias. Pelayo retired into a narrow valley flanked by mountains, from which it was impossible to attack on a broad front, and thus easy to defend. Pelayo may have had no more than 300 men with him at this point.
Alqama eventually arrived at Covadonga and sent forward an envoy to convince Pelayo to surrender. He refused, so Alqama ordered his best troops into the valley to fight. The Asturians opened fire from the slopes of the mountains, and then at some point Pelayo personally led some of his soldiers out into the valley. They had been hiding in a cave, unseen by the Moors. The Christian accounts of the battle claim that the slaughter among the Moors was great, a claim that in this case is probably true. Alqama himself fell in the battle, and his soldiers fled from the battlefield.
In the aftermath of Pelayo's victory, the people of the conquered villages of Asturias now emerged with their weapons and killed hundreds of Alqama's fleeing troops. Munuza, learning of the defeat, organized another force and gathered what was left of the survivors of Covadonga. At some later date, he confronted Pelayo and his now greatly augmented force near the modern town of Proaza. Again Pelayo won, and Munuza was killed in the fighting. And although the Muslims in their own histories called Pelayo and his men "thirty wild donkeys", they never again seriously challenged the independence of the Kingdom of Asturias.
The site of the Battle of Covadonga: http://www.almargen.com.ar/sitio/seccion/historia/covado/enol.html