Comment by YZF

The S&P 500 can and will crash at some point. It has and it will. That's part of the lifecycle of market psychology. We go through cycles of over valuation and under valuation. It's true there are many forces with interest in keeping the markets up but there have always been and it's always crashed because once the psychology changes there is no amount of intervention that can keep the market up.

If you are invested for the long term then just don't think about it. If you want to diversify a little then go for it - slowly. Also keep in mind your US index fund is full of international companies anyways.