We (the US) supported the UK in this overthrow, so blame the British.  If it wasn't for the Brits we wouldn't have done it.  At the time, Iran wasn't in our "sphere of influence."  If anyone is unaware of this event and wants to begin to learn more, as always, start with the Wikipedia entry:  https://en.wikipedia.org/wiki/1953_Iranian_coup_d'%C3%A9tat