After the conquest of Rayy and Central Persia, Umar ordered the conquest of Azarbaijan. The province of Azarbaijan lay to the west of the Caspian Sea, and was so called because of large number of fire temples therein.
Umar appointed Hudheifa to the command of the campaign against Azarbaijan. Hudheifa first marched to Zanjan. Here the local garrison put up resistance but they were overpowered and the city was carried by assault.
From Zanjan the Muslim forces proceeded to Ardabeel the capital of the province. The Persians did not offer any resistance and surrendered on the usual terms of Jizya. From Ardabeel the Muslim forces marched northward along the western coast of the Caspian Sea. There was a confrontation at Bab which was an important port on the Caspian Sea. The Muslims scored a victory.
At this stage Hudheifa was recalled. The Persians launched a counter attack, and the Muslims abandoned their forward posts in Azarbaijan Umar now sent the expeditionary forces to Azarbaijan, one led by Bukair bin Abdullah and the other by Utba bin Farqad.
The contingent under Bukair had their first confrontation with the Persians at Jurmizan. The Persians were commanded by Isandiar. In the battle which was quite severe, the Persians were defeated and their commander Isandiar was captured alive. Isandiar asked Bukair, "Do you prefer war or peace." Bukair said that the Muslims preferred peace. Isandiar thereupon said, "Then keep me with you till I can help you in negotiating peace with the people of Azarbaijan". There were many forts in the hills. The Persians went to these hills and shut themselves in the forts. The Muslims captured the entire area in the plains.
The other Muslim forces under Utba bin Farqad had their confrontation with a Persian force commanded by Bahram a brother of Isandiar. The Persians were defeated with heavy loss and Bahram fled away.
When Isandiar came to know that Bahram had beer defeated, he waited on Utba and negotiated for peace. In the peace pact that was drawn up, the people of the region agreed to accept the Muslim rule and to pay Jizya.
The Muslims amassed considerable booty. That was distributed among the soldiers. Utba b. Farqad carried the state share to Madina. Along with the other gifts that he carried were loads of 'Halwa', a speciality of Azarbaijan. When Umar tasted the Halwa, he said that it was mos delicious and sweet. He had the 'Halwa' distributed among all persons in Madina. All those who ate it felt that it was a sweet from the Heaven.