It depends on the circumstance. If Russia for some reason invades Turkey, by NATO charter that would be an invasion of the US. But it is not really an invasion of the US, so the US may elect to not do anything about it.
Exactly.
If the US even starts using conventional force against Russia, then the Russians may reply with tactical nuclear weapons. US hits back with it's own tactical nuclear weapons and the Russians may be tempted to launch an ICBM strike on US to stop them from interfering. The risk of escalation to full blown nuclear war is so great that the US would most likely sit it out and just condemn Russia.
During the cold war, the US sent 300,000 soldiers to West Germany to try to convince Western Europe that it would be prepared to fight to the death against the old Soviet Union. No such force exists in Turkey as a reassurance.