Open Question: Why did American CIA and British M16 overthrow the democracy in Iran in the 1950's? ?

I heard that because the Iranian parliament voted to naturalized oil and this might hurt the British so they along with the US decided to overthrow him and installed an friendly oil dicatotor. Why did American CIA and British M16 overthrow the democracy in Iran in the 1950's and replace with friendly dictator? Why they did for that for? I wonder why?