Whether it is defensive in nature all depends on the number of troops. If its a few thousand, that's defensive, if its fifty thousand, that's invasion level.
Why the hell can't the Saudis defend themselves? They've got enough money from oil revenues to purchase any amount of weapons systems they want.
Don't forget that America charges for their services. It took Britain ages to pay off America for their help in defending us in WW2
If we joined in, it would nice little earner! It's just that we haven't much to offer in the way of hardware
But even if we had the hardware it might not be a good investment if we ended up with loads of Muslims in Britain engaged in acts of terrorism.
Then again, Iran are already sponsoring terrorism beyond the Middle East.
The big question is whether in an invasion of Iran by the West whether there are enough countries in the Middle East prepared to step up to the plate. Because if they're not publicly supportive they are bound to become our foes, if they aren't already.
If Saudi dissociates itself from the planned American incursion/invasion, if I was the US I would call it off.
How much oil does America still need oil from the Middle East?