Keeping us energy dependent on the Middle East. Lots of jobs lost in oil-producing states, which post fracking/shale = lot of damage to the economy nationally.
This will also make Saudi Arabia unstable as they go broke in a year or two with prices at this level.
Low oil prices are devastating to Texas, Louisiana and America and it is not by accident, it is because of the Obama War on Oil.
As an aside, large military movements use enormous quantities of oil and can tap storage and reserves quickly. In such a scenario, the nation with the most oil can have some advantages. Also, expectedly, China has bought much into oil production here and there around the world over some time. Today, there was news that China is establishing a military base in Africa.