The 2015 Presidential Election

On 28 March 2015 Muhammadu Buhari of the All Progressives Congress (APC) won Nigeria's presidential election, defeating the incumbent, Goodluck Jonathan of the People's Democratic Party. According to Wikipedia, the election ended sixteen years of PDP rule, and Buhari, sworn in on 29 May 2015, became the first opposition candidate to win a presidential election since independence in 1960. The election had been postponed by six weeks because of violence by Boko Haram in the north-east.

Muhammadu Buhari had stood for president three times before. According to his Wikipedia article, he was the candidate of the All Nigeria Peoples Party in 2003 and 2007, losing in 2007 to Umaru Yar'Adua, and of the Congress for Progressive Change, a party he helped found, in 2011, when he came second to Goodluck Jonathan with 32 per cent of the vote (Wikipedia's history of Nigeria). Before the 2015 election four opposition parties merged to form the All Progressives Congress, and in December 2014 Buhari became its presidential candidate.

Jonathan's government was criticised for its handling of the Boko Haram insurgency, including the kidnapping of the Chibok schoolgirls in 2014, and for corruption; Wikipedia's history of Nigeria describes the high level of corruption as a decisive factor in the election. The vote, originally set for mid-February, was postponed by six weeks because of Boko Haram violence in the north-east; observers described the election as fair, and Buhari, then 72, won on his fourth attempt (Wikipedia).

Buhari was sworn in on 29 May 2015, ending sixteen years of government by the People's Democratic Party. He was re-elected in February 2019 and served until 29 May 2023 (Wikipedia).

Wikipedia's history of Nigeria notes that Jonathan conceded defeat, and that this was the only time in the Fourth Republic that voters refused to re-elect an incumbent president.