Iran attacked US base in Iraq with ballistic missiles. My educated guess: The US will hit selected Iranian targets in Iranian territory such as military facilities, possibly air force bases, oil production facilities, oil refineries, uranium enrichment facilities, highways etc.
The reaction of the US will depend on whether the Iranian attacks killed any US soldiers. If they did, the US administration will have to make the decision to retaliate Iranian attacks.
This scenario contains lots of risks in terms of regional stability in the Middle East. If Israel and/or Saudi Arabia joins the US forces in case of a military operation against Iran, the situation will get more complex and dangerous.
Since Russia plays the game to push the US and the West back off the Middle East (in Syria) and co-operates closely with Tehran for this goal, Moscow might use this chance to back Iran (most likely diplomatically). However, a regional war might occur as well (less likely).
Putin is now in Turkey, meeting with Erdogan. Did Iran informed Russia (and perhaps Turkey) before the missile attacks? Pure speculation! But I am sure this topic will shadow all others in Putin-Erdogan talks.
Russia must be concerned about the future of the Turkish regime right now, for I guess this new situation will re-shuffle the cards! And Washington might need a clearly pro-American administration in Turkey now.
I guess this is something which Erdogan cannot give Washington since Erdogan co-operates closely with pro-Russian Eurasianists who control Turkish military. He cannot stay a minute in power without their support. And they don’t want Turkey backs the US in this war.
Nevertheless, is Washington really wants there will be a regime change in Turkey.