- Rahul Gandhi, Sonia Gandhi attended the swearing-in ceremony
- Chief Ministers including Mamata Banerjee, Chandrababu Naidu were present
- Once rivals, Mayawati and Akhilesh Yadav also shared stage
Here's your 10-point cheatsheet to this big story:
Mr Kumaraswamy, who was sworn in along with his deputy G Parameshwara, said he will take the trust vote on Friday. The JDS-Congress alliance leaders had brought its legislators to the venue since they have been isolated in city hotels to ensure they aren't poached before the test of strength in the assembly.
Having turned his swearing-in ceremony into a show of opposition unity, the Chief Minister suggested that a lot was riding on the success of the alliance government. "I have to be very careful," he said, about running the coalition government.
Of the many non-BJP leaders who had travelled to Bengaluru for the swearing-in, Mr Kumaraswamy said they weren't just here for him. "The leaders have not come here to support me but to give a message that there will be a big change in 2019," he said.
The event was high on symbolism. The swearing-in ceremony was the first joint public appearance by Uttar Pradesh's bitter rivals-turned-allies, Akhilesh Yadav and Dalit powerhouse Mayawati shared the stage. Akhilesh Yadav warmly greeted her and later, stood next to her, waving to the cheering crowds. Mayawati, who has promised to continue her alliance for next year's Lok Sabha elections, had a pre-poll alliance with JDS in Karnataka.
The Mayawati-Akhilesh Yadav team in Uttar Pradesh and the Congress-JDS alliance in Karnataka stands for the kind of new friendships that HD Kumaraswamy would like to see in the rest of the country.
The many photo-ops and the bonhomie among the parties on stage, however, is no indicator that other rival non-BJP parties too will come together. West Bengal's Mamata Banerjee and CPM's Sitaram Yechury were there and at one awkward moment, even came face to face. They politely greeted each other, shook hands and walked away.
The Congress-JDS were quick to form an alliance after the state election threw up a hung assembly, with Mr Kumaraswamy as their head. Together they have 117 members, enough for an outright majority.
The Congress will have 22 ministers and the JDS 12, Congress' KC Venugopal said, adding that they would be sworn-in after the floor test slated for Friday.
The post of the Speaker and the Deputy Speaker will be evenly divided - the Speaker will be from the Congress, his deputy will be from the Janata Dal Secular. The Congress has named Ramesh Kumar for the job, the JDS is yet to name its candidate.
BJP's BS Yeddyurappa, who was Karnataka chief minister for just two day before he quit ahead of a trust vote on Saturday, today predicted the Congress-JDS alliance won't last "even three months".