Obama might have made people think this is shaky, but the US will back Israel in any conflict where Israel is too out gunned, but as far as that goes, it has its own nuclear deterrent and proven military in such a fight without us.

Next, as long as we have a significant level of troops in Iraq, and perhaps even if we pull out of a very stable Iraq, we'll guarantee its defense.

We also have a long term stake in Kuwait.

We also saw in Iraq War I (The Gulf War), if the Saudi oilfields are threatened, we'll feel we have to join in.

I'd have to think about it, but there really isn't a nation Iran is likely to attack, if it so desired, that we would not feel a significant need to help. Israel might be an exception but only because they have proven they don't need it.

I'd think we'd stand a better chance long term of being drawn into trouble we might want to avoid by our extending NATO to places like Poland and other Eastern European nations once part of the Soviet bloc.

Sort of like a Bosnia/Kosovo situation but with an automatic trigger if Putin's Russia becomes the norm long term and we have meanwhile contracted a security arrangement with those former Soviet allies…

The event with Georgia and Russia recently is a sign of this danger.

In short, US troop presence and the necessity of protecting the world's oil supply – coupled with our long-term close relationship with Israel – likely makes helping defend against Iran "mandatory" –

— but Russia can field a bigger and better military and threaten nations we really don't have an overwhelming interest in protecting – making offering such a shield to them more questionable – I'd think – than to Iraq or Kuwait.