Any President would be a fool not to prepare for any contingency. As to Israel attacking Iran, well, the USA has a commitment to protect the existence of the State of Israel from any attack intended for her destruction. President Obama has reaffirmed that many time. And while Israel as a sovereign nation can make it own decision and handle its own consequences. it would be foolish to think that any sane leader would place its people in danger to support any Israeli action.
U.S.-led coalition conduct 23 air strikes against ISIS in Iraq and Syria (Reuters)
from the article: U.S. has military plan should Iran conflict erupt, says air force chief