The Nigerian government launched a "police action" to retake the secessionist territory. The war began on July 6, 1967 when Nigerian Federal troops advanced in two columns into Biafra. Nigeria's army offensive was through the north of Biafra led by Col. Shuwa and designated as 1 division. The division was made up of mostly northern officers. The right-hand Nigerian column advanced on the town of Nsukka which fell on July 14, while the left-hand column made for Garkem, which was captured on July 12. At this stage of the war, other regions of Nigeria (the West and Mid-West) still considered the war as a confrontation between the north (notable Hausas) and the east (notable Igbos).
However, the Biafrans responded with an offensive of their own when on July 9, the Biafran forces moved west into the Mid-Western Nigerian region across the Niger River, passing through Benin City, until they were stopped at Ore just over the state boundary on August 21, just 130 miles east of the Nigerian capital of Lagos. The Biafran attack was led by Lt. Col. Banjo. They met little resistance and the Mid-West was easily taken over. This was due to the arrangement and agreement between Federal government and the East that all soldiers should be returned to their regions to stop the spate of killings in which Igbos soldiers had been major victims. The soldiers that were supposed to defend Mid-West were mostly mid-west Igbos and were in touch with their eastern counterpart. Gen. Gowon responded by asking then Col. Muritala to form another division (2 division) to expel Biafrans from mid-west, defend Biafra's west and attack Biafra from the west as well. Col. Muritala later became military head of state. As Nigerian forces were to retake the Mid-West, the Biafran military administrator declared the Republic of Benin on September 19.
Although Benin City was retaken by the Nigerians on September 20, the Biafrans succeeded in their primary objective by tying down as many Nigerian Federal troops as they could. Gen. Gowon also launched an offensive from Biafra's south from the delta to riverine area using the bulk of Lagos Garrison command under Col. Adekunle (black scorpion) to form 3 division which latter changed to the 3rd marine commandos. Recruitment into the Nigeria Army increased with Biafra's offensive to the west mostly among other southern ethnics especially Yoruba and Edo people. Four battalions of the Nigerian 2nd Infantry Division were needed to drive the Biafrans back and eliminate their territorial gains made during the offensive. But the Nigerians were repulsed three times and lost thousands of troops as they tried to cross the Niger during October.
However reorganization of the Nigerian forces, the reluctance of the Biafran army to attack again, and the effects of a naval, land and air blockade of Biafra led to a change in the balance of forces.
The Swedish eccentric, Count Carl Gustaf von Rosen, also led a flight of MiniCOINs in action; his BAF (Biafran Air Force) consisted of three Swedes and two Biafrans.
The Nigerians then settled down to a period of siege by blockading Biafra. Amphibious landings by the Nigerian marines led by Major Isaac Adaka Boro captured the Niger Delta cities of Bonny, Okrika and Port Harcourt on July 26, and the port of Calabar on October 18 by elements of the Nigerian 3rd Marine Commando Division. In the north, Biafran forces were pushed back into their core Igbo territory, and the capital of Biafra, the city of Enugu, was captured by Nigerian forces belonging to the 1st Infantry Division on October 4. The Biafrans continued to resist in their core Igbo heartlands, which were soon surrounded by Nigerian forces.