The Second Martial Law
The second martial law was imposed on March 25, 1969, when President Ayub Khan abrogated his own constitution and handed over power to the Army Commander-in-Chief, General Agha Mohammad Yahya Khan.  On assuming the presidency, General Yahya Khan acceded to popular demands by abolishing the one-unit system in West Pakistan  and ordered general elections on the principle of one man one vote.
General Yahya's regime made no attempt to frame a constitution. The expectations were that a new constituent assembly would be set up by holding a free and fair election. In order to hold the proposed elections, President Yahya Khan promulgated a Legal Framework Order on March 30, 1970 that also spelled out the fundamental principles of the proposed constitution and the structure and composition of the national and provincial assemblies.
In December, 1970 elections were held simultaneously for both the national and five provincial assemblies. By any criteria, elections were free and fair. There was no interference from the government; it maintained strict neutrality showing no favor or discrimination for or against any political parties. The members of the ruling council of ministers were debarred from participation in the elections. There were no allegations of rigging of the elections as is often alleged in elections held in the countries of the third world.
But the results of the first and the last general elections in united Pakistan were simply disastrous from the standpoint of national unity and demonstrated the failure of national integration. There was not a single national party in the country which enjoyed the confidence of the people of Pakistan, both East and West Pakistan. Two regional parties -- the Awami League under the leadership of Sheikh Mujibur Rahman in East Pakistan (now Bangladesh)-- won 160 out of 162 seats allotted for East Pakistan. But in West Pakistan it could not secure a single seat and the percentage of votes secured by the Awami League in the four provinces of West Pakistan were: 0.07 (Punjab), 0.07 (Sindh) 0.2 (NWFP) and 1.0 (Baluchistan).
The Pakistan People's Party (PPP) under the leadership of Zulfikar Ali Bhutto won 81 out of 138 seats for West Pakistan. But the PPP did not even dare to set up a candidate in East Pakistan. The remaining 57 seats of West Pakistan were shared by seven parties and there were fifteen independent candidates. The PPP emerged as the single largest party in West Pakistan with majorities in Sindh and the Punjab; and the National Awami Party together with their political ally, Jamiat-ul Ulema-i-Islam, JUI, (of Maulana Mufti Mahmood got clear majorities in Baluchistan and the North West Frontier Province. None of the West Pakistani political parties, like the PPP, could win a single seat in East Pakistan. The religious question played little or no part in the elections. There can be no doubt that in East Pakistan the principles which won the consensus of opinion was the single basic notion of autonomy.
The Awami League (AL) had fought the elections on the basis of their six points formula, which committed them to restructure the existing federal system in order to ensure maximum political autonomy for East Pakistan. Under this formula, only two portfolios -- Foreign Affairs and Defence -- would be retained by the central government. The PPP, on the other hand, was not willing to dilute the authority of the central government in-spite of assuring full provincial autonomy for all the provinces of Pakistan. The NAP and JUI coalition sided with the Awami League so that they might obtain maximum autonomy for their own provinces, i.e., Baluchistan and the NWFP.
The election results truly reflected the ugly political reality: the division of the Pakistani electorate along regional lines and political polarization of the country between the two wings, East and West Pakistan. In political terms, therefore, Pakistan as a nation stood divided as a result of the very first general elections in twenty-three years of its existence.
Thus the general elections of 1970 produced a new political configuration with three distinct centers of power:(i) the AL in East Pakistan: (ii) the PPP in Sindh and the Punjab; and (iii) the NAP-JUI in Baluchistan and the NWFP. At the top of all this was the fourth center of power, the armed forces with their spokesman, Yahya Khan.
There were two major claimants of power: Sheikh Mujibur Rahman and Zulfikar Ali Bhutto. Both were inordinately ambitious and unscrupulous politicians. "Both flourished on negative appeals to the illiterate voters of Pakistan, one by whipping up regional feeling against Punjabi domination and the other by whipping up militant national feelings against India. Neither had any constructive or positive approach."  Mujib was apparently more interested in creating a separate state for Bengalis, Bangladesh since he had no trust in the ruling elite of West Pakistan. He also seemed to have entered into a secret deal with India for the secession of East Pakistan. India, thanks to its consistently and persistently hostile attitude towards Pakistan, was gladly ready to help Mujib in his secession plan.
On the other hand, Bhutto was more interested in getting power, no matter whether in a united or divided Pakistan. In fact he realized that in a united Pakistan, he had little chance of becoming either prime minister or president. According to GW Choudhury, "he realized from his discussions with Bhutto before and after the 1970 elections that if he had to make a choice between the two 'Ps (power or Pakistan), he would choose the former. He was more interested in getting a 21-gun salute as the head of the state than in the maintenance of the unity of Pakistan." 
Negotiations were held between January and March 1971 between the two major regional leaders - Mujib and Bhutto - and the ruling military government under President Yahya Khan. But the tripartite negotiations for an agreed federal or even a confederal constitution was a dismal and total failure. It is now a well-known fact that the negotiations in Dacca with Mujib were a smoke screen for gaining time for Yahya Khan to airlift supplies and military personnel to Dacca for subsequent military operations.  Under the Legal Framework Order, the President was to decide when the Assembly was to meet. Once assembled it was to frame a new constitution within 120 days or stand dissolved. On 13th February, 1971, the president announced that the National assembly was to meet at Dacca on 3rd march. By this time the differences between the main parties to the conflict had already crystallized.
On December 22, 1970 the Secretary of the Awami League, Tajuddin Ahmed, claimed that his party having won an absolute majority had a clear mandate and was quiet competent to frame a constitution and to form a central government on its own. Sheikh Mujibur Rehman declared on January 3, 1971 that his party would not frame a constitution on its own, even though it had a majority. He refused, however, to negotiate on the Six Points, saying that they were now public property and no longer negotiable.
The crux of the conflict was that the majority party in the west, led by Bhutto, was convinced that a Federation based on the Six Points would lead to a feeble confederation in name only. At best it would lead to a feeble confederation and at worst it would result in the division of the country into two states. These fears were evidently shared by the military leaders in the west, including President Yahya Khan who had publicly described Sheikh Mujibur Rehman as the 'future Prime Minister of Pakistan' on January 14, 1971.
Bhutto announced on February 15 that his party would not attend the National Assembly unless there was 'some amount of reciprocity' from the Awami League. Sheikh Mujib replied at a press conference on February 21, asserting that 'Our stand is absolutely clear. The constitution will be framed on the basis of the Six Points'. He also denied that the Six Points would leave the central government at the mercy of the provinces and contended that they were designed only to safeguard provincial autonomy.
On February 28, Bhutto demanded that either the 120-day limit for the national Assembly be removed or the opening session be postponed, declaring that if it was held on March 3 as planned, there would be a general strike throughout West Pakistan. President Yahya Khan responded next day by postponing the Assembly meeting to March 25. The postponement of the National Assembly came as a shattering disillusionment to the Awami League and their supporters throughout East Pakistan. It was seen as a betrayal and as proof of the authorities of the West Pakistan to deny them the fruits of their electoral victory. This resulted in the outbreak of violence in East Pakistan. The Awami Leaguelaunched a non-cooperation movement and virtually they controlled the entire province.
The National Assembly, however, could not even meet on March 25 due to widespread disturbances in East Pakistan where the army moved in on march 26, 'to control the situation' or launching ruthless atrocities against the innocent people. The civil disobedience movement later developed into a war of national liberation fully backed by the Indian army. As a result, Pakistani forces had to surrender to the Indian Army, and almost over 93,000 military personnel were taken as prisoners of war on December 16, 1971. Thus ended an important era of the largest Muslim state, Pakistan. A new and smaller Pakistan emerged on 16 December 1971.
Demoralized and finding himself unable to control the situation, Yahya Khan surrendered power to Bhutto who was sworn-in on December 20, 1971 as President and the Chief Martial Law Administrator. Among the generals, once again, there seemed to be a deep cleavage about the future course of action. The generals close to Yahya Khan wanted him to stop transferring power to the then Commander-in-Chief, General Abdul Hamid Khan.The hawkish generals, such as Gul Hassan and Air Marshal Rahim Khan, however are rumored to have practically forced Yahya Khan at gun point to transfer power to Bhutto, who thus became the first civilian to take as Chief Martial Law Administrator and President of Pakistan.  Such was the background of events against which the Bhutto era of Pakistan's political history began on December 20, 1971.
On Bhutto's role in the cessation of East Pakistan it is argued that as the National assembly was to sit at Dacca, Bhutto prevented members of the National Assembly from West Pakistan from going to Dacca, by threatening them that he would break the legs of those who went to Dacca. He also tore to pieces the Polish Resolution in the United Nations which had proposed that the two belligerent states should revert to their original positions before the war had begun. "You on that side and I on this side," is another expression which Bhutto is alleged to have addressed to Mujib.