First up, THAAD from America:
The U.S. State Department has approved the possible sale of a THAAD anti-missile defense system to Saudi Arabia at an estimated cost of $15 billion, the Pentagon said on Friday, citing Iran among regional threats.
An earlier version of the article said that the sale wouldn't affect the military balance in the region. That I'm sure refers to the Saudi-Israeli balance. THAAD anti-ballistic missile defenses won't have much of an effect since Israel has means other than ballistic missiles to hit Saudi Arabia--if the warming Saudi-Israel relations focused on Iran suddenly chill.
And I'm sure deployment of missiles and radar will focus on an arc starting at Iran and stretching south to Yemen.
Apparently Saudi Arabia isn't confident that Iran won't have nuclear missiles in the near future.
For the Israel front, just in case, Russia has stepped in:
Saudi Arabia has agreed to buy Russian S-400 surface-to-air missile systems, Saudi-owned al-Arabiya television reported on Thursday. ...
It said the procurement was “based on the assurance of the Russian party to transfer the technology and localize the manufacturing and sustainment of these armament systems in the Kingdom”, but provided no timeframe.
Russia doesn't care if the Saudis shoot down air threats from Israel, Iran, or Yemen.
The Saudis certainly want a friendly channel to foes Iran and Syria, I imagine, which this deal will help create.
And this could telegraph a Saudi unhappiness with a Trump administration refusal to cancel and walk away from the Iran nuclear deal, and in place of that "outreach" to "reset" Iran, commit to an anti-Iran coalition. We'll see.
Still, the silver lining for America and Israel is that the Saudis will get technology transfers which will likely make its way to us, giving us more insights into the S-400 system. Assuming Russia sends the most advanced version and not an export version.
And given that Saudi Arabia is a potential launching pad for any military intervention in the region, having air defenses protecting Saudi airfields, ports, and oil infrastructure is surely important to America.