The battle, for which one of the most detailed accounts of the entire Umayyad era survives in the History of al-Tabari, halted or reversed Muslim expansion into Central Asia for a decade.

The losses suffered by the Khurasani army also led to the transfer of reinforcements from the metropolitan regions of the Caliphate, which in the long term weakened the Umayyad regime and helped bring about its collapse twenty years later in the Abbasid Revolution that began in Khurasan.

The difficult security situation at the time is illustrated by the fact that Junayd needed an escort of 7,000 cavalry after crossing the Oxus River, and that he was attacked by the Türgesh khagan Suluk while riding to link up with the army of his predecessor, Ashras al-Sulami, who in the previous year had advanced up to Bukhara in a hard-fought campaign.

This left him seriously short of men when, in early 731, the Türgesh laid siege to Samarkand and appeals for aid arrived from the city's governor, Sawra ibn al-Hurr al-Abani.

Despite the opinion of the army's veteran Khurasani Arab leaders, who counselled that he should wait to reassemble his forces and not cross the Oxus with fewer than 50,000 men, Junayd resolved to march immediately to Samarkand's rescue.

[21][22] As the historian Hugh N. Kennedy writes, "when the nomad [Türgesh] allied with the local Iranian princes, they provided what was perhaps the fiercest opposition the early Muslim armies ever encountered".

[23] Supported by troops from the rulers of Sogdia, Shash, and Ferghana, the Türgesh attacked the Umayyad army in the pass, two days after they had left Kish (a Friday), six farsakhs (c. 24 km or 15 mi) from Samarkand.

Junayd, who had placed himself in the centre to direct the battle, then joined the ranks of the Azd, who greeted him with hostility: their standard-bearer is reported to have told him "If we win, it will be for your benefit; if we perish, you will not weep over us.

The Arabs initially met the Türgesh attack on horseback, but as their casualties mounted, Junayd's herald ordered them to dismount and fight on foot, crouching down behind the trenches and forming a spear wall.

Leaving behind a small garrison, Sawra led 12,000 men out of Samarkand and with the help of a local guide managed to reach within a farsakh (roughly 5–6 km or 3.1–3.7 mi) of Junayd's force by crossing over the mountains.

Junayd again resolved to meet them in battle and managed to inflict some defeats on the Türgesh in early November and raise the siege of Bukhara, which he entered on the day of Mihragan.

Judging by the numbers of replacements ordered sent to or levied in Khurasan in the aftermath of the battle, Blankinship estimates the Arab losses at between 25,000 and 30,000, and that "probably not more than fifteen thousand Khurasani troops were left alive".

[41] Although the Türgesh also suffered heavy casualties – Ibn A'tham gives the unverifiable figure of more than 10,000 dead[42] – the Arab losses at the Battle of the Defile led to a rapid deterioration of the Umayyad position in Central Asia.

Junayd remained as governor of Khurasan until his death in early 734, but by this time the Muslims had lost control of everything north of the Oxus save for Bukhara, Kish, and the region of al-Saghaniyan.

Al-Tabari also reports the words – albeit possibly a later addition – of another Khurasani to Junayd before the battle: "It used to be said that certain of the troops of Khurasan would perish at the hands of a luxury-loving man from the Qays.

[33][47] Blankinship observes that: [A]fter the Day of the Defile, many Khurasani tribal surnames never again appear as part of the army in Khurasan, leading one to suppose they had been annihilated or their men had given up fighting.

[39]The subsequent period in Khurasan was turbulent, with revolts and anti-Umayyad agitation among the local Khurasani Arabs, necessitating the introduction of 20,000 Syrian troops into the province, in addition to the Iraqis sent in after the Battle of the Defile.

Only in 739–741, after the Türgesh Khaganate collapsed following the murder of its leader Suluk, was the new governor of Khurasan, Nasr ibn Sayyar, able to largely restore the Caliphate's position in Transoxiana, and extend Muslim control again up to Samarkand.

[48][49] In the aftermath of the setbacks at the battles of the Defile, Marj Ardabil, and other similar disasters, the Umayyad government was forced to take urgent measures to reinforce the buckling frontiers of the empire.