The US can decimate a road convoy including the murderous most senior Iranian military commander; the Iranians can use ‘commensurate’ force and lob handfuls of missiles on US bases in Iraq. The Iraqi’s can demand the US remove its personnel from Iraq. The US can demand payment in full for the billions spent on infrastructure in Iraq since the war “ended.” And…not a lot happens.
The US may or may not have calculated that Iran’s response would be measured. Iran did calculate that its response would be sufficiently measured that no major conflict would ensue, Iraq got to pacify its peoples demand that its sovereignty and the honour of its new part-time friend would be respected. The US got to rattle its sabre. Its allies got to speed up repatriation of their forces and the Arab world, generally, got to confirm its pet hate. The Israelis probably have that unfulfilled feeling. For the most part, while tension increases the guns will remain silent…there is too much to lose all round