Though the Nigerian military recently made gains against Boko Haram militants, Jonathan, 57, never dealt a decisive blow against the Islamic extremists, who declared a caliphate in the country's northern regions. The militants killed 10,000 people last year and forced about 1.5 million to flee for southern Nigeria and neighboring countries. They have kidnapped hundreds of people, including 219 schoolgirls who have yet to be found nearly a year after their abduction.

Wednesday, Buhari warned Boko Haram he was coming after it. "No doubt this nation has suffered greatly in the recent past, and its staying power has been challenged to its limits, chief among them the insurgency of Boko Haram," he said at Abuja's international conference center, according to the Associated Press. "Boko Haram will soon know the strength of our will and commitment to rid this nation of terror. … We shall spare no effort until we defeat terrorism."

Supporters say the president-elect — who takes office May 29 — will succeed against the extremists where Jonathan failed because of his background.

"Experience matters a lot in the fight against insurgency," said Stanley Ibe, 32, a teacher. "As a retired general, Buhari is better placed to tackle the menace head-on — far better than his predecessor who has no military background."

Buhari rose through the ranks to the title of military governor in the nation's northeast in the 1970s. He then served as the country's leader from 1983 to 1985 before being ousted in a military coup.

During those periods, Buhari took a zero-tolerance approach to crimes, including minor infractions such as littering and not standing properly in line to board buses. He instituted draconian media laws, cracked down on journalists and jailed opponents, giving officials near-unlimited arresting powers.

At the same time, he tackled bribery and graft, jailing 500 prominent public servants and members of the business community for the offenses. He was also known for not using state coffers for personal gain. In 1983, he forced Chadian soldiers to retreat after they tried to annex Nigerian islands. Buhari confronted insurgencies, including an Islamic religious sect stirring trouble in 1981.

Buhari served as a federal oil commissioner in the late 1970s. He reorganized the nation's petroleum organizations and expanded the country's industry, constructing more facilities and pipelines.

His ouster as the country's head of state in 1985 came amid tumbling oil prices and a sinking economy because of measures he implemented such as banning imports, which led to the closure of businesses and spiking unemployment. The downturn was used to justify the coup against him. Buhari responded by accusing elements of the elite of trying to evade corruption probes.

"Buhari has the willpower to confront corruption and insecurity," said Ochiaka Ugwu, political analyst and public affairs commentator based in Abuja. "But he must go about it democratically."