At the outset of his reign misfortune stalked the footsteps of Yazdjurd. He came to power at the time when the Muslims were poised to burst across the horizon, and embark on their career of conquests. In 633 C.E. the Muslims under Khalid b Walid conquered a greater part of Iraq then under Persian rule.
When Khalid b Walid was transferred to the Syrian front, the Persians were able to recapture most of their territories in Iraq. Under Umar, the Muslims once again launched the attack against the Persians. At the battle of Qadissiyia in 636 C.E. the Persians suffered a decisive defeat. This battle marked the beginning of the end of the Sassanian empire. Yazdjurd vacated Madain the capital of the Persians in Iraq. The Persians suffered another defeat at the battle of Jalula. Thereafter Yazdjurd fell back on Isfahan. The Persians met another reverse at the battle of Nihawand. This battle decided the fate of Persia and Yazdjurd. By 642 C.E. it was clear that the days of the Sassanian empire were numbered and it could not survive.
After the battle of Nihawand the Muslims advanced to Isfahan, and Yazdjurd left for Rayy. At Rayy he realised that the Governor was disloyal to him. With a few followers, Yazdjurd left for Persepolis.
In 643 C.E. the Muslims invaded Fars, Yazdjurd left Persepolis for Kirman. When the Muslims captured Kirman, Yazdjurd fled to Seestan. When the Muslims continued their pursuit and overpowered Seestan, Yazdjurd fled to Merv.
His overwhelming misfortunes made his people lose confidence in his leadership. The people of Merv wanted him to make peace with the Muslims and thereby save whatever little was left of the empire. To this course he did not agree. The people of Merv seized his treasure and drove him away from the city.