Why was Margaret Thatcher called the Iron Lady?

Margaret Thatcher liked to act tough. She had several nicknames, most of them unflattering.

Thatcher was called the Iron Lady because of her tough approach in dealing with her opposition, especially in the war with Argentina.
And now, she has a Street with his name in Madrid:http://www.tucalle.com/cartel_de_calle-de-margaret_thatcher_en_madrid.htm