Sayf al-Islam al-Badr (as Muhammad now became), not yet 20, was clearly able to patch up speedily any misunderstandings with his father, for in late 1949 he was appointed his deputy over Hodeida, the important port on the Red Sea. He was also made Minister of the Interior.
Al-Badr played a prominent role in quelling the revolt against Imam Ahmad in 1955 led by Ahmad's brother Sayf al-Islam Abdullah and afterwards was declared Crown Prince. During the remaining period of Imam Ahmad's rule he held the post of Minister of Foreign Affairs and from 1958 he was also the Imam's deputy over Sanaa. In 1959 he was put in complete charge of the Yemen for a few months during Imam Ahmad's absence in Italy for medical treatment. An assassination attempt on the life of Imam Ahmad in March 1961 left the latter gravely crippled and in October Sayf al-Islam al-Badr took over effective control of the government.
On 19 September 1962 Ahmad died in his sleep, al-Badr was proclaimed Imam and King and took the title of al-Mansur, but a week later rebels shelled his residence, Dar al Bashair, in the Bir al-Azab district of Sanaa and set up a republic.
Al-Badr had, when Crown Prince, like most young Arab leaders of his generation, been a great admirer of the Egyptian President Jamal Abd al-Nasir and had even arranged during his father's absence in Italy for Egyptian experts to come and help modernize the Yemen in all fields, including the military. His father moreover had incorporated Yemen into the United Arab Republic of Egypt and Syria, which then became the United Arab States. It is thus ironic that the Yemen revolution of 26 September 1962 was largely instigated and planned by Egyptians and that without a massive Egyptian presence in the Yemen for five years afterwards the Yemen Arab Republic could never have survived.
Although the revolution had announced to the world that al-Badr had died beneath the rubble of his palace, he had in fact managed to escape unhurt and set out to the north. As he proceeded on his journey the tribes rallied round him pledging him their unconditional allegiance as Amir al-Mumineen ("Prince of the Faithful"). These tribes were zealous Zaydi Shia for whom unstinted loyalty to an imam from the Ahl al-Bayt (the descendants of the Prophet) was a fundamental obligation of their religion. A few days later he held a press conference over the border in south-west Saudi Arabia. His uncle Sayf al-Islam al-Hasan, who had been abroad and had been proclaimed Imam at the news of al-Badr's alleged demise, immediately gave allegiance to him together with all the princes of the Hamid al-Din family. Soon the entire tribal confederation of Bakil along with most of Hashid who occupied the central and northern highlands of the Yemen and who had been Zaydis for centuries joined enthusiastically the cause of the Imam and the princes to fight the revolutionary regime.
During the bloody civil war which continued for eight years al-Badr, like his cousins, played a vital role. He lived alongside his men the life of a warrior, sharing with them every deprivation and hardship. He set up his headquarters in various places in the scenically spectacular mountainous north-west Yemen, on Jebal Qara, for instance, in the region of Hajur al-Sham and at al-Muhabisha high up above the Tihama plain. These HQs situated in caves fitted out with every basic facility deep in the mountainside were nevertheless constantly under the threat of Egyptian bombardment from the air. In 1967 al-Badr left his HQ at Mabyan near Hajjah for Taif in Saudi Arabia, where he stayed until the end of the war.
Al-Badr was a man of great courtesy, kindness and personal charm. He loved dearly the Yemeni people and was essentially a man of peace. He said he would never allow a terrible civil war to rage once again in his country. He assisted his father in defending his father's control over North Yemen from two rebellious brothers in 1955.
Ahmad named al-Badr crown prince. In 1956 he forged connections and signed agreements during a tour to Soviet bloc countries. During his father's trip abroad for medical treatment in 1960 al-Badr introduces a number of reforms in Yemen which his father had promised. His father annulled these upon his return. In 1962 Imam Ahmad died, and al-Badr succeeded him. Among al-Badr's first actions was to grant amnesty to political prisoners.
On September 26 of that year Abdullah as-Sallal, who al-Badr had appointed commander of the royal guard, staged a coup, and declared himself president of the Yemen Arab Republic. Al-Badr escaped to the north of North Yemen, and rallied tribes that support him in opposition to Sallal. Fighting erupted between the two groups, starting the North Yemen Civil War. Al-Badr started getting support from Saudi Arabia, while the republicans received support from Egypt.
In 1970, despite the fact that territorially most of the Yemen remained under the control of al-Badr and the Hamid al-Din family, Saudi Arabia, which had been the principal opponent of the Sanaa regime, recognized the Yemen Arab Republic and other nations like the United Kingdom swiftly followed suit.
Stunned by Saudi Arabia's recognition of the republican regime which had been negotiated without any consultation with him whatsoever, al-Badr refused to stay any longer in Saudi Arabia and demanded that he be permitted to leave the kingdom immediately. He went to England, where he lived quietly in a modest house in Kent, only going abroad to visit the holy cities of Mecca and Medina and to call on relatives and friends in that part of the world.