Mosaddeq, Iran, USA and Britain in 1953

[Faceless]

I'm trying to understand the nature of the 1953 coup. Its effects were deep cutting and long lasting. The United States stops a democratic nation because of (originally) the oil nationalisation from the AIOC. So why did the USA get involved to reinstate the Shah of Iraq? To start with, wasn't this largely a British affair? I've heard that they were fearing a drift to Communism. It might not be much of an excuse but was it true?
Excuse my ignorance and thanks in advance. :)

[Hampton]

CIA Documents (http://www.payk.net/politics/cia-docs/payk-0.1/iran-cia-main.1.tex.txt)
Article by Masoud Kazemzadeh, Ph.D (http://64.224.31.98/53Coup/Articles-2003/day_democracy_died-essayMK.htm)
Long essay that will make your eyes bleed (http://iran.sa.utoronto.ca/coup/web_files/markcoup.html)
Shot account (http://www.iranchamber.com/history/coup53/coup53p1.php)
Insiders view of the coup (http://www.politrix.org/foia/cia-stuff/cia-iran-lob.htm)
Roots of Middle East Terror (http://www.rense.com/general40/roots.htm)
The Biography of Dr. Mohammad Mossadegh (http://www.jebhemelli.org/Mosadegh/English-Mosadegh.htm)