Question: what was the decisive for Crassus's disastrous defeat in the most crushing defeats in Roman history ?

full_context: Document 1: Battle of Ctesiphon (363): The Battle of Ctesiphon took place on May 29, 363 between the armies of Roman Emperor Julian and an army of the Sasanian Empire (during Shapur II's reign) outside the walls of the Persian capital Ctesiphon. The battle was a Roman victory, although Julian was killed following the subsequent Battle of Samarra and the Roman forces found themselves unable to continue their campaign as they were too far from their supply lines.
Document 2: Battle of Maranga: The Battle of Maranga occurred in 363 AD, shortly after the Battle of Ctesiphon. The Romans repelled a Sasanian attack while sustaining minimal losses. However, the army's lack of supplies continued to threaten the army, and soon afterwards the emperor Julian was killed at the Battle of Samarra.
Document 3: Military of the Sasanian Empire: The Sasanian army (Middle Persian "Spâh-ī Sāsānīg") was the primary military body of the Sasanian armed forces, serving alongside the Sasanian navy. The birth of the army dates back to the rise of Ardashir I (r. 224–241), the founder of the Sasanian Empire, to the throne. Ardashir aimed at the revival of the Persian Empire, and to further this aim, he reformed the military by forming a standing army which was under his personal command and whose officers were separate from satraps, local princes and nobility. He restored the Achaemenid military organizations, retained the Parthian cavalry model, and employed new types of armour and siege warfare techniques. This was the beginning for a military system which served him and his successors for over 400 years, during which the Sasanian Empire was, along with the Roman Empire and later the East Roman Empire, one of the two superpowers of Late Antiquity in Western Eurasia. The Sasanian army protected "Eranshahr" ("the realm of Iran") from the East against the incursions of central Asiatic nomads like the Hephthalites and Turks, while in the west it was engaged in a recurrent struggle against the Roman Empire.
Document 4: Rorarii: Rorarii were soldiers who formed the final lines, or else provided a reserve thereby, in the ancient pre-Marian Roman army. They may have been used with the "triarii" in battle near the final stages of fighting, since they are recorded as being located at the rear of the main battle formation. (Note that the saying "Going to the Triarii" means that something has gone to the bitter end – as in reached the final line.)
Document 5: Publius Aquillius Gallus: 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.
Document 6: Battle of Thymbra: The Battle of Thymbra was the decisive battle in the war between Croesus of the Lydian Kingdom and Cyrus the Great of the Achaemenid Empire. Cyrus, having pursued Croesus into Lydia following the drawn Battle of Pteria, met the remains of Croesus' partly disbanded army in battle on the plain north of Sardis in December, 547 BC. Even though Croesus' army was reinforced with many new men, Cyrus utterly defeated it, despite being outnumbered more or less 2:1. This proved decisive, and after the 14-day Siege of Sardis, the city and possibly its king fell, and Lydia was conquered by the Persians.
Document 7: Battle of Carrhae: 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.
Document 8: Marcus Licinius Crassus: 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.
Document 9: Sagittarii: 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.
Document 10: 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.
Document 11: Byzantine army: The Byzantine army or Eastern Roman army was the primary military body of the Byzantine armed forces, serving alongside the Byzantine navy. A direct continuation of the Roman army, the Eastern Roman army maintained a similar level of discipline, strategic prowess and organization. It was among the most effective armies of western Eurasia for much of the Middle Ages. Over time the cavalry arm became more prominent in the Byzantine army as the legion system disappeared in the early 7th century. Later reforms reflected some Germanic and Asian influences – rival forces frequently became sources of mercenary units e.g.; Huns, Cumans, Alans and (following the Battle of Manzikert) Turks, meeting the Empire's demand for light cavalry mercenaries. Since much of the Byzantine military focused on the strategy and skill of generals utilizing militia troops, heavy infantry were recruited from Frankish and later Varangian mercenaries.
Document 12: Julian's Persian War: Julian's Persian War, or the Perso-Roman War of 363, was the last undertaking of the Roman emperor Julian, begun in March 363. It was an aggressive war against the Persian Empire ruled by the Sassanian king Shapur II. Shapur is believed to have expected an invasion by way of the Tigris valley. Julian sent a detachment to join with his ally Arshak II of Armenia and take the Tigris route. Meanwhile, with his main army he advanced rapidly down the Euphrates valley, meeting only scattered opposition, and reached the walls of the Persian capital Ctesiphon where he met and defeated the Persian army at the Battle of Ctesiphon (363). Unable to take the city, and with a faltering plan of campaign, He was misled by Persian spies into burning his fleet and taking a disadvantageous route of retreat in which his army was constantly harassed and his progress crawled to a halt.
Document 13: Surena: 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.
Document 14: Centurion: A centurion (Latin: "centurio" ; Greek: κεντυρίων , "kentyríōn", or ἑκατόνταρχος , "hekatóntarkhos") was a professional officer of the Roman army after the Marian reforms of 107 BC. Most centurions commanded groups of centuries of around 80 men but senior centurions commanded cohorts or took senior staff roles in their legion. Centurions were also found in the Roman navy. In the Byzantine Army, they are also known by the name kentarch (κένταρχος , "kentarchos"). Their symbol of office was the vine staff, with which they disciplined even Roman citizens protected from other forms of beating by the Porcian Laws.
Document 15: Harquebusier: The harquebusier was the most common form of cavalry found throughout Western Europe during the early and mid 17th century. Early harquebusiers were characterised by the use of a form of carbine, called a "harquebus". In England, harquebusier was the technical name for this type of cavalry, though in everyday usage they were usually simply called 'cavalry' or 'horse'. In Germany they were often termed "ringerpferd", or sometimes "reiter", in Sweden they were called "lätta ryttare".
ground_truth: mounted archers