A former arch-rival of Indian Prime Minister Narendra Modi aligned with the Hindu nationalist leader on Thursday to form a new government in Bihar, one of the country's most populous states.
The outspoken Nitish Kumar took the oath as Bihar's chief minister just hours after standing down in protest at corruption allegations against his last coalition partner.
The link up between Kumar's Janata Dal United party and the prime minister's right-wing Bharatiya Janata Party (BJP) further tightens Modi's grip on power ahead of national elections in 2019.
Kumar had been considered a potential challenger to Modi for India's top job.
The BJP, which this month easily won the presidential election for the first time, now rules 17 of India's 29 states either directly or in alliance with regional parties. 
Bihar, with a population of 100mn people but impoverished and frequently scandal-tainted, has seen a titanic power struggle in recent years.
In 2013 then-chief minister Kumar pulled his party out of a 17-year-old alliance with the BJP over Modi's appointment as party leader.
He had called Modi a "fascist" and "Hitler" over his alleged anti-Muslim and hardline Hindu views. 
After stepping down in 2014 as chief minister after a poor showing in elections, Kumar forged a "grand alliance" with another ex-rival and former chief minister, Lalu Prasad Yadav, to thwart Modi in the following year's state polls.
The alliance, which also involved the Congress party, was seen as a possible prototype for a national bid to stop Modi getting re-election.
Yadav's son Tejashwi Yadav became a deputy chief minister under the deal, but is now under investigation by federal investigators over an illegal land deal. He has denied the charges. His father was jailed and is banned from elected office over a separate 2013 fraud scandal.
After several calls for Tejashwi Yadav to stand down, Kumar resigned late on Wednesday saying it was "impossible" to carry on.
Kumar will face a floor test in two days to prove his new majority.
"I promise that I will stay committed to the people of Bihar," Kumar told reporters after taking oath in state capital Patna. 
Modi congratulated Kumar for taking over as the chief minister, a day after he commended him for taking a stand against corruption.
Yadav's party said they would also stake a claim to form the government and accused Kumar of playing into a BJP trap.
Congress vice-president Rahul Gandhi accused Kumar of "cheating" the people and the alliance.
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