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In the movie Bajirao Mastani, Baajirao Ballad Maratha Peshwa (Bramhin by birth) performs a Yagna after every battle he fights, in order to clear the sin of fighting war as it's a sin to fight if you are not a kshatriya.
So this means that any bramhin can fight in war by performing specific yagna, and even a Bramhin or Vaishya can do kshatriya karma in special circumstances? What did Lord Parshurama do in order to fight like that of kshatriya? Similarly any kshatriya can do bramhin karma or vaishya karma if yes then is the method same?
What is that yagna which allows one to do different varna karma? What are other methods to perform another varna karma, like kshatriya doing bramhana/vaishya karma visa versa?