On December 6th 1992 a mob of Hindutva activists destroyed the Babri Masjid, a 16th century mosque in the city of Ayodhya in UP. Some dispute over the site existed after a claim in the mid-19th century that the mosque was built on the birthplace of Rama ("Ram janmabhoomi"). In 1949 the mosque was broken into and Hindu idols placed inside; Jawaharlar Nehru got the mosque sealed to prevent communal tension and the likely gains for the Hindu Mahasabha that would result.
The issue was revived in the 1980s by the VHP and Bajrang Dal, and soon became a key rallying point of the Sangh Parivar. It became clear that the BJP could make large political gains out of the communal polarisation created by the movement. In 1990, BJP leader L. K. Advani mobilised kar sevaks to converge on the Babri Masjid, supposedly to offer prayers; however the UP government prevented them reaching the mosque citing the communal violence being created.
No doubt as a consequence of the communal polarisation, the BJP came to power in UP in 1991 and, in December 1992, with the support of chief minister Kalyan Singh, the kar sevaks managed to reach the mosque. Despite a commitment to the Indian Supreme Court by the organisers, the mosque was torn down, by an estimated crowd of 150,000 people.
Communal violence erupted around India with an estimated 2000 people, mainly Muslims, dying. Since 1992, the issue of building a temple at the site of the demolished mosque has frequently been revived by the Sangh Parivar. For example, many of those killed in the fire on the Sabarmati Express were kar sevaks returning from Ayodhya. In November 2019, the Supreme Court settled the title dispute over the land in favour of the Hindu parties, essentially awarding the land to the VHP, which had torn down the Mosque. Narendra Modi himself referred to the decision as "a new dawn for us".
References:
• Hindu Nationalism: A Reader Edited by Christophe Jaffrelot, Princeton, 2007.
• The Use and Misuse of History and Archaeology in the Ayodhya Dispute, Thomas Van Damme.
• Some resources on Communalism Watch, South Asia Citizens Web, and Wikipedia.