Turkey (NATO ally) can go to war without NATO approval or support, but if attacked NATO must respond to defend Turkey... (i am not mistaken on this, right?)
And NO, Nato includes the French and the UK, totalling a huge number of professional soldiers, several aircraft carriers, nuclear weapons capability, and nuclear submarines... Even if the US was busy focusing somewhere else (like, oh i don't know, let's say the South China Sea) the NATO allies could take effective measures against Iran or Syria... At least outside of Iran (as it is the ability to project force we're talking about here, Iran could project force into syria, or provide soft support, while the UK and France combined could counter that much Iranian power, neither of them could project enougy force to invade Iran proper... They'd have to nuke Iran to guarentee surrender.
Meanwhile what Russian involvement might look like is totally up in the air. Gas prices, of cutting off supply to Europe once winter hits... They can defend their borders (as the georgia invasion proved) and they're willing to use force, but i think they'd only provide soft support and political pressure to keep NATO allies from creating new colonies/puppets...