We saw Al Qaeda do this quite effectively in 2001. In spite of the massive security efforts of the American government, America still has lots of soft targets both in and outside of America's borders. These will be the targets of choice for a determined enemy. America spent three trillion dollars and lost 5000 military dead during their unnecessary advanture in Iraq. This would be peanuts in a war with Iran.
At least five killed, 13 injured in Macedonia storm (DPA)
from the article: Iran official: U.S. cannot stop us from cutting off world oil supply