Is the attack on Shia militias in Iraq a prelude to an attack on Iran?

I am currently reading "Bitter Friends Bosom Enemies, Iran, the US and the Twisted Path to Confrontation" by Barbara Slavin. I am only about 1/4 of the way through the book but so far it has provided a lot of interesting background on the current stand-off between the US and Iran; with one thing especially jumping out at me.

During a discussion of a possible military response to Iran's nuclear program a lecturer at the Naval Postgraduate School said that one of the considerations that was holding the US back was the possibility of Iranian backed Shiite militias in Iraq retaliating against US troops.

Well if the militias are disarmed it becomes a lot harder for them to retaliate doesn't it?

Couple that with the announcement today of the promotion of David Petraeus to Command of CENTCOM, where he would have responsibility for Iran and not just Iraq as he currently does and Commander MNF-I, and it makes me wonder what exactly is going on.