What was Zia's role during the 1971 war?
I expect someone else to give you an elaborate answer to your question. My short answer is as follows:
Zia was a Major in the Paksitan Army working in the East Bengal Regiment. He was stationed at the Chittagong Cantonment when the liberation struggle started. Important AL leaders were in flight to save their lives, and Sheikh Mujib HAD NOT declared independence of Bangladesh. So, people were disoriented.
In lieu of this, the Bangali Police in the Razarbaag HQ in Dhaka raised against the much superior Pakistan Army when the latter attacked on the early morning of 26th March, 1971. Police had only .303 rifles to repulse machine gun attacks by the military. Many hundreds of them perished. At least 10,000 people were killed in Dhaka on that night.
Since there was no explicit declaration of independence from Sk. Mujib or any other leaders, it made East Bengal Regiment troops, East Pakistan Rifles' Bangali troops, Police, Ansars (militias) and population quite disoriented. They just did not know what the Bangali political leadership intended to do.
This was a time when Yahya Khan had already moved about 25,000 to 30,000 Bangali troops to west Pakistan and under the plan of Pakistan Army the remainder in the east were to be systematically killed after being disarmed.
Since there was no central command structure against the occupation of Pakistan Army, therefore, it was going to be a one-sided slaughtering of Bangali armed men. In such a critical time, Chittagong AL leaders were trying to seek supports from Bangali troops in the Cantonement.
And Major Zia was the person who, upon request and after giving due consideration to all the aspects, came forward and declared for an independent Bangladesh. He made two different declarations. The first one said of a Bangladesh Provisional Governmet under his command. But, when the Chittagong AL leaders protested that it cannot be accepted, he changed the statement something as follows:
"I, Major Ziaur Rahman, hereby declare the independence of Bangladesh on behalf of our great leader Bangabandhu Sheikh Mujibur Rahman."
This much-awaited declaration worked as a magic to the population and the armed men. When they found that East Bengal Regiment troops are supporting the independence, they started to moblize themselves with whatever weapons, including bamboo sticks, they could master to fight against the occupation army.
It also happened that before Pakistan army could capture and kill the Bangali army, EPR troops, Police and Ansars, this declaration allowed them to distance themselves from the west Pakistani troops and thus save their lives. Only because of participation by these Bangali troops it was easy for us to fight. Ordinary Muktis were under their command.
So, Ziaur Rahman's cotribution was a major factor that the initial political resistance could be transformed into an armed struggle to liberate the country. Without this timely declaration, most of the Bangali troops would have been killed, and even if then AL wanted to continue the war it would have to fully depend upon India.
However, population would not have supported that kind of dependence upon India at that initial stage. So, it is very difficult to forecast an outcome of our liberation war if Zia's timely declaration was not made. It can also be said that the war could not have progressed without EBR troops leading it.
Sorry, it was supposed to be a short answer, but became a little lengthy. However, you will admit that it is shorter than a hundred page book on this topic.
By the way, Zia was one of the nine Sector Commanders of the Liberation War, and afterwards became the President of Bangladesh. He was a good administrator, at par with President Ayub Khan.
if you know what I mean 
