The JD(U) chief ended his almost two years' alliance with Lalu Prasad and the Congress on Wednesday. He was sworn in by officiating state Governor Keshari Nath Tripathi. The BJP's Sushil Modi was also sworn in and got back his earlier charge of deputy chief minister.
Other ministers will be sworn-in after Nitish will win a trust vote in the legislative assembly on Friday. His new National Democratic Alliance (NDA) government has called a special session of the assembly to prove its majority.
"Whatever decision we have taken will be in the interest of Bihar and of its people. It will ensure development and justice. This is a collective decision. I ensure that our commitment is towards the people of Bihar," Nitish told reporters after the swearing-in.
Immediately after the ceremony, his JD(U) announced it would support the BJP-led NDA government at the centre, in both Houses of Parliament. The two parties were in alliance for 17 years until 2013, when Nitish walked out of the NDA fold after Narendra Modi was made the BJP's prime ministerial candidate.
Nobody from the RJD or the Congress attended the swearing-in ceremony, protesting the "denial of chance" to the RJD. However, Lalu Prasad has decided to appeal in the Supreme Court against the state governor's decision to invite Nitish to form the government.
"I'll go to the apex court and appeal that as the single largest party, the RJD should have been invited first to form the government. I'm in touch with (eminent lawyer) Ram Jethmalani. You know about the S R Bommai case?" Prasad told scribes in Ranchi.
He said, Nitish "not only cheated but also backstabbed the people of Bihar". "The match was fixed. It was all part of the strategy that (Sushil) Modi made corruption charges against me and my family," said Prasad.
The Congress also reacted sharply to Nitish dumping it. "People do anything for their selfish motives. There is no principle, no credibility. They can do anything for power...He (Nitish) had joined hands with us in the fight against communal forces but for his personal politics, joined hands with those against whom he had fought," said party Vice-President Rahul Gandhi.
The JD(U) is also facing a revolt within its ranks — as seniors are unhappy at joining hands with the BJP. Former party president Sharad Yadav, Ali Anwar and Virendra Kumar, all Rajya Sabha members, have openly expressed their dissent. "We are astonished by this decision. We were never told by the party leadership about the move. Sharadji has asked for a couple of days to discuss and ponder. He also refuted rumours about him joining the Union Cabinet," said Ali Anwar after he met Yadav in Delhi.
The NDA alliance had given a list of 132 MLAs (of 243 in the assembly) to the governor around midnight on Wednesday. These were 71 of the JD(U), BJP (53), RLSP (2), LJP (2), HAM (1) and three independents. While, the RJD has 80, the Congress 27 and the CPI-ML three.
 "Bihar Governor Keshari Nath Tripathi administers oath to JD(U) President Nitish Kumar as Bihar Chief MInister at Raj Bhawan, in Patna.")