"The Danube took the crown of Rome" - The Battle of the Gates of Trajan in 986

17 August 986, the young Byzantine warrior emperor Basil II suffered the only defeat in his long career at the hands of the future Bulgarian Tsar Samuil at the Battle of the Gates of Trajan.
“Even if the sun would have come down, I would have never thought that the Moesian [Bulgarian] arrows were stronger than the Avzonian [Roman, Byzantine] spears. ... And when you, Phaethon, descend to the earth with your gold-shining chariot, tell the great soul of the Caesar: The Danube took the crown of Rome. The arrows of the Moesians broke the spears of the Avzonians." (John Geometres)

A 14th century illustration from the Mannasses Chronicle, 
showing Bulgars and Byzantines at war.    

Actually, the First Bulgarian Empire was supposed to have collapsed in 971 under the combined pressure of the Kievan Rus, the Hungarians and that of their old arch-enemies, the Byzantines. After the Emperors Nikephoros Phokas and his nephew and successor John Tzimiskes had successfully driven the Rus out of Bulgaria, the place was put under Roman military administration and the Bulgarian Tsar Boris II was brought back to Constantinople, ritually de-invested of his imperial regalia and that was that. Or so the Byzantines thought. But there was still life in the old dog, as the saying goes, and in 976 the new Roman emperor, 18 years old Basil II was all of a sudden faced with a serious revolt in the western parts of the allegedly subdued Bulgarian provinces. And with the Fatimids breathing down his neck in the East, the Roman Empire was in dire troubles again.

An image of one Bulgaria's greatest heroes, Tsar Samuil* 

Four brothers led the rebellion, called the Cometopuli since they descended from the Comita, a count, Nicola, probably the local ruler of Sredets, the mighty fortress that was to become Sofia a couple of hundred years later and consequently young Emperor Basil tried to quell the uprising at a stroke and marched 30,000 men towards the Cometopuli’s stronghold. And the impregnable fortifications of Sredets proved too much of a challenge for the Romans. After a couple of weeks they withdrew with their tails between their legs. But things were to get even worse for Basil and his men. On their return march through the Sredna Gora mountains, at a pass known as the Gates of Trajan, just a couple of miles before the rough terrain ended and the safety of the Upper Thracian Plains began, Samuil, the youngest of the four Cometopuli, sprang his trap.

Basil II, Bulgaroktonus, from a 11th century manuscript

Most of the seasoned Byzantine troops were deployed on their Eastern front fighting the Fatimids and only Basil’s Armenian bodyguards held when Samuil’s Bulgarians pounced on the Roman army from all sides. The young emperor and his Armenians cut their way out with heavy losses but the day was lost and most of the Roman army was vanquished at the Gates of Trajan. For the next two decades Samuil, soon to be Tsar of the Bulgarians, was master of the Eastern Balkans and raided deep into Thrace and Greece. Basil II, in the meanwhile never lost a battle again, fought the Fatimids to a standstill and returned to Bulgaria with a vengeance in 1002 when a full-scale war broke out between the Bulgarian and the Byzantine Empire that ended in a disaster for Samuil in 1014. And while Basil II gained the charming moniker Bulgaroktonus, Slayer of Bulgars, Tsar Samuil passed into the local legends as one of their greatest heroes.

* the image was found on http://bnr.bg/en/post/100423839/samuel-bulgarias-cain-who-had-his-reasons

And more about the Battle of the Gates of Trajan can be found on: