Why US call backs its citizen to return to country?

Still the brusting crisis of CAA and NRC issue has not yet seen a solution in india, there we have another international issue to address. The tension arose between US and Iran after Iran's Quds forces attack of Baghdad embassy. US started to deploy its army forces in Iraq and today US killed Qassem Soleimani,the head of Iran's elite Quds Force, and Abu Mahdi al-Muhandis, the deputy commander of Iran-backed militias known as the Popular Mobilization Forces, or PMF through air stike. The Pentagon confirmed the strike, saying it came "at the direction of the president". 

Soleimani and al-Muhandis's deaths are a potential turning point in the Middle East and are expected to draw severe retaliation from Iran and the forces it backs in the region against Israel and US interests.

So, now United State ask its citizen living Iraq to leave the country and this tickles the question that "does US gonna go for war with Iran?"