That is to let Iran know that a nuclear attack attempted on Israel by Iran, Syria or hezbollah or Hamas terrorists will result in the complete nuclear annihilation of every metropolitan area in Iran, all military facilities and all infrastructure. That threat must be made as real as possible so they understand it. Iran will then turn their attention to the Sunni oil Gulf nations which will really upset the US as opposed to Iran taking a direct anti-Israel position. When the Saudis / Kuwaitis are threatened, the US will act.
Red Cross: Scale of sexual violence in South Sudan is unprecedented (Reuters)
from the article: Russia: Deal reached with Iran on nuclear plant construction