Source, in full
This quote is widely attributed to Margaret Thatcher on various websites, and also appears in a number of books, including The Concise Columbia Dictionary of Quotations, Columbia University Press (1989), ed. Robert Andrews, p. 320 : ISBN 0231069901. 9780231069908 , but without any further source information such as date, location or any other context.
Notes from Wikiquote
One valid Thatcher quote which may be the basis for the version above appears in the Second Carlton Lecture (‘Why Democracy Will Last’), delivered at the Carlton Club, London (November 26, 1984) : Mr. Chairman, each generation has to stand up for democracy. It can’t take anything for granted and may have to fight fundamental battles anew. You know that marvellous quotation from Goethe : ‘That which thy fathers bequeathed thee
Earn it anew if thou would possess it.’ | Thatcher also expressed this thought in a Speech to Atlantic Bridge (May 14, 2003), delivered at the St. Regis Hotel, New York City : My friends, every generation has to fight anew the battle for liberty.