Question: To which event where three Roman legions were distroyed is the phrase The German Forest sometimes used to refer to?

Context paragraphs:
German Forest: The German Forest (German: "Deutscher Wald" ) was a phrase used both as a metaphor as well as to describe in exaggerated terms an idyllic landscape in German poems, fairy tales and legends of the early 19th century Romantic period.  Historical and cultural discourses declared it as the symbol of Germanic-German art and culture, or as in the case of Heinrich Heine or Madame de Staël, as a counter-image of French urbanity.  It was also used with reference to historical or legendary events in German forests, such as Tacitus' description of the Battle of the Teutoburg Forest or even the nature mysticism of the stylized Germanic national myth, the "Nibelungenlied" as the history of its multi-faceted reception shows.

Arminius (Bruch): Arminius (op.  43) is an oratorio by the German composer Max Bruch.  Bruch wrote the work between 1875 and 1877 during the consolidation of the newly founded German Empire.  He picked the story revolving around Arminius and the Cherusci-led defeat of three Roman legions in the Teutoburg Forest in 9 A.D., which served as a German national myth from the 16th to the early 20th century.

Arminius: Arminius (German: "Hermann"; 18/17 BC – AD 21) was a chieftain of the Germanic Cherusci tribe who famously led an allied coalition of Germanic tribes to a decisive victory against three Roman legions in the Battle of the Teutoburg Forest in 9 AD.  An auxiliary lieutenant to the Roman general Publius Quinctilius Varus, Arminius used his knowledge of Roman tactics to ambush and destroy the legions.

SPQR: SPQR is an initialism of a Latin phrase "Senātus Populusque Rōmānus" ("The Roman Senate and People", or more freely as "The Senate and People of Rome"; ] ), referring to the government of the ancient Roman Republic, and used as an official emblem of the modern-day comune (municipality) of Rome.  It appears on Roman currency, at the end of documents made public by inscription in stone or metal, in dedications of monuments and public works, and was emblazoned on the vexilloids of the Roman legions.

Rufio (officer of Caesar): Rufio was an officer of the Roman general and statesman Julius Caesar.  In 47 BC he was appointed by Caesar commander-in-chief of the three Roman legions that were stationed in Egypt.

Roman auxiliaries in Britain: The overall size of the Roman forces in Roman Britain grew from about 40,000 in the mid 1st century AD to a maximum of about 55,000 in the mid 2nd century.  the proportion of auxiliaries in Britain grew from about 50% before 69 AD to over 70% in c. 150 AD.  By the mid-2nd century, there were about 70 auxiliary regiments in Britain, for a total of over 40,000 men.  These outnumbered the 16,500 legionaries in Britain (three Roman legions) by 2.5 to 1.  This was the greatest concentration of auxilia in any single province of the Roman Empire.  It implies major continuing security problems; this is supported by the (thin) historical evidence.  After Agricola, the following Emperors conducted major military operations in Britain: Hadrian, Antoninus Pius, Constantius I and Septimius Severus.

Roman commerce: The commerce of the Roman Empire was a major sector of the Roman economy during the early Republic and throughout most of the imperial period.  Fashions and trends in historiography and in popular culture have tended to neglect the economic basis of the empire in favor of the lingua franca of Latin and the exploits of the Roman legions.  The language and the legions were supported by trade while being at the same time part of its backbone.  Romans were businessmen and the longevity of their empire was due to their commercial trade.

Massacre in the Black Forest: Massacre in the Black Forest (German: "Hermann der Cherusker - Die Schlacht im Teutoburger Wald" ) is a 1967 historical drama film set on the northeast frontier of the Roman Empire facing the area of Germania in A.D. 9.  The film centers on Hermann, a chieftain of the Cherusci Germanic tribe (in what is now Hesse), who drew three Roman legions into an ambush in the Teutoburg Forest, known as the Battle of the Teutoburg Forest.

Battle of the Teutoburg Forest: The Battle of the Teutoburg Forest (German: "Schlacht im Teutoburger Wald" , "Hermannsschlacht", or "Varusschlacht"), described as the Varian Disaster ("Clades Variana") by Roman historians, took place in the Teutoburg Forest in 9 CE, when an alliance of Germanic tribes ambushed and decisively destroyed three Roman legions and their auxiliaries, led by Publius Quinctilius Varus.  The anti-Roman alliance was led by Arminius, a Germanic officer of Varus' auxilia who had acquired Roman citizenship and had received a Roman military education, thus enabling him to deceive the Roman commander methodically and anticipate the Roman army's tactical responses.

Publius Quinctilius Varus: Publius Quinctilius Varus (46 BC Cremona, Roman Republic – Sept. 9 AD near Kalkriese, Germany) was a Roman general and politician under the first Roman emperor Augustus.  Varus is generally remembered for having lost three Roman legions when ambushed by Germanic tribes led by Arminius in the Battle of the Teutoburg Forest, whereupon he took his own life.

Supporting paragraphs: German Forest Battle of the Teutoburg Forest
Answer: Battle of the Teutoburg Forest