If the US were to pull away from Norway, I'm sure at least the UK would step in. The UK has nukes, and they need Norwegian oil, so it seems very unlikely to me that they would allow Russia to invade or occupy Norway. I can't imagine that France would be happy with such an incursion, either, and they also have nukes.
Having said that, with the evolving economic and political changes in the world, any country that relies primarily on others for its defense is leaving themselves open to risk.
If Norway were invaded, the terrain there is such that the country would be impossible to hold IF the residents fought back.
I live in New Zealand, a country of about the same size and population as Norway, and with similar rich natural resources. We're fortunate to not have an neighbors with common borders, but there is still a feeling of concern, particularly with regard to China, but even from currently friendly Australia and US.