Question: what was the decisive for Crassus's disastrous defeat in the most crushing defeats in Roman history ?
Context: Surena or Suren or Sourena (died 53 BC) was a Parthian spahbed ("General" or "Commander") during the 1st century BC.  He was a member of the House of Suren and was best known for defeating the Romans in the Battle of Carrhae.  Under his command Parthians decisively defeated a numerically superior Roman invasion force under the command of Marcus Licinius Crassus.  It is commonly seen as one of the earliest and most important battles between the Roman and Parthian empires and one of the most crushing defeats in Roman history. Marcus Licinius Crassus Frugi (flourished 1st century BC & 1st century) was a Roman nobleman of consular rank who lived during the Roman Empire.  Frugi’s mother was an unnamed Roman woman, while his father was consul and governor Marcus Licinius Crassus.  Frugi’s adoptive paternal grandfather was consul and general Marcus Licinius Crassus.  Crassus was the grandson of triumvir Marcus Licinius Crassus and the last known direct descendant of his grandfather.  He had a sister called Licinia who married the consul, Lucius Calpurnius Piso; their son, Gaius Calpurnius Piso was a conspirator against the Roman emperor Nero. The Byzantine–Bulgarian wars were a series of conflicts fought between the Byzantines and Bulgarians which began when the Bulgars first settled in the Balkan peninsula in the 5th century, and intensified with the expansion of the Bulgarian Empire to the southwest after 680 AD.  The Byzantines and Bulgarians continued to clash over the next century with variable success, until the Bulgarians, led by Krum, inflicted a series of crushing defeats on the Byzantines.  After Krum died in 814, his son Omurtag negotiated a thirty-year peace treaty.  In 893, during the next major war, Simeon I, the Bulgarian emperor, defeated the Byzantines while attempting to form a large Eastern European Empire, but his efforts failed. The 1968–69 Toronto Maple Leafs season was the Toronto Maple Leafs 52nd season of the franchise, 42nd season as the Maple Leafs.  Although the Maple Leafs made the playoffs, they were swept in the quarter-finals by Boston, suffering two crushing defeats at Boston Garden. Sagittarii (Latin, plural form of "Sagittarius") is the Latin term for archers.  The term "sagittariorum" in the title of an infantry or cavalry unit indicated a specialized archer regiment.  Regular auxiliary units of foot and horse archers appeared in the Roman army during the early empire.  During the Principate roughly two thirds of all archers were on foot and one third were horse archers.  Mercenary foot archers already served with the Roman republican army, but horse archers were only introduced after the Romans came into conflict with eastern armies that relied heavily on mounted archery in the 1st century BC.  Most notably the Parthians, whose mounted archers were decisive for Crassus's disastrous defeat in the Battle of Carrhae.  In the early 1st century CE horse archers were already in widespread use and even supported Roman campaigns against the Germanic tribes in northern Europe. The Battle of Carrhae ] was fought in 53 BC between the Roman Republic and the Parthian Empire near the town of Carrhae.  The Parthian Spahbod ("General") Surena decisively defeated a numerically superior Roman invasion force under the command of Marcus Licinius Crassus.  It is commonly seen as one of the earliest and most important battles between the Roman and Parthian empires and one of the most crushing defeats in Roman history. Publius Aquillius Gallus was a tribune of the plebs in 55 BC.  With his colleague Gaius Ateius Capito, Aquillius Gallus opposed the "Lex Trebonia" and the plans regarding proconsular commands for Crassus and Pompeius.  Crassus's war against Parthia resulted in one of the worse defeats ever suffered by a Roman army, the Battle of Carrhae. Yuriy Trubetskoy ("Jurij Trubetsky", "Jerzy Trubecki", "Jurij Trubiacki", "Юрий Петрович Трубецкий ", "Юрій Петрович Трубецький ", "Juri Petrovitz Troebieskoy"; ca 1643 – 12 July 1679 buried in Troitsky monastery) was a Ruthenian Prince, boyar of the Trubetsky family.  In 1657, Prince Jurij Trubetsky went to Moscow with his uncle Aleksandr Yurievitch Trubetzkoy (?  - Poland after 1657).  Stolnik in Moscow in 1660, he was given a boyar title by Tsar Alexis I of Russia in 1673.  In the late 17th century, the weakened Republic of Both Nations under King Jan III Sobieski in alliance with the forces of Leopold I, Holy Roman Emperor, dealt the crushing defeats to the Ottoman Empire; Jerzy Trubecki was the Polish wojewoda of Kiev Voivodeship, 1673.  He hold the scepter at the coronation of Fedor Alexeevitch in 1676. Marcus Licinius Crassus ( ; c. 115–53 BC) was a Roman general and politician who played a key role in the transformation of the Roman Republic into the Roman Empire.  Amassing an enormous fortune during his life, Crassus is, excepting Augustus Caesar, considered the wealthiest man in Roman history. Rholes or Roles (Ancient Greek Ῥώλης) was a Getae chieftain in Scythia Minor (modern Dobruja) mentioned by Cassius Dio in his "Roman History".  According to Dio, he helped Roman general Marcus Licinius Crassus defeat the Bastarnae, and when he visited Octavian, he was treated as "a friend and ally" for his support for the Romans.  Later he sent for Crassus to help in his conflict with Getae chieftain Dapyx.  These events have been dated to 31-27 BC.
Answer: mounted archers