“The reasonable man adapts himself to the world: the unreasonable one persists to adapt the world to himself.
Therefore all progress depends on the unreasonable man.” ― George Bernard Shaw, Man and Superman
Still, they are to an extent friendly to the US or at least not overtly hostile, can't say the same about Iran.
And allowing Iran to cut off the red sea, and potentially the Persian gulf and therefore holding the flow of oil under siege is unacceptable not just to the US, but much of the world.
It would be an act of war, from most of the world.
Iran signed the NPT in 1968 and is therefore bound by its XI Articles unless it withdraws as North Korea did in 2003.
Four states - India, Israel, Pakistan, and South Sudan - never signed the NPT.
Crimea is illegally occupied by Russia
It will be interesting to see if the Saudi's actually decide to send troops across the border, or more likely if they decide to airlift troops into Aden, and engage in ground operations. They performed very poorly the last time around and had to lean heavily in Jordanian troops to support their efforts. It's been about five years but I'm skeptical that their capabilities have improved significantly. Lack of professionalism, under strength units, and low morale have historically plagued the Saudi land arm. Though it is eminently possible that they learned their lessons and have taken pains to bring more units up to true professional standards.