The Escalation Ladder of Nuclear Weapons

Russia, Syria and Iran are teaming up right now to try and push the US out of the Middle East. The Pentagon just sent a squadron of F-16s and F-35s out to patrol the Strait of Hormuz. It would be very tempting to take out a Russian plane but then what's next? And it's that escalation ladder. Where does it end? And what does it trigger in terms of a use of a nuclear weapon?