If America and/or Israel were to attack Iran, the world's oil prices would skyrocket overnight. This is because the oil supply is controlled by speculators who will react very swiftly to even the most remote threat to their interests. More than anything America fears a major blow to their economic recovery. Raising oil prices will raise prices in every sector. Iran would likely let the attackers gain full credit for this result and would not muddy the waters by taking their own justifiable measures.
Western-backed Syrian rebels say kill foreign Islamic State (Reuters)