It is most definitely irrational, because there is no rational reason why the US has that position.
Egypt's and Saudi Arabia's stance toward Israel has softened partly because of their relationship to the U.S., and partly because of the bifurcation of regional interests--the enemy of my enemy is my friend.
Turkey, notably, has recently vacillated between strong support of Israel (including bilateral--U.S. not involved--strategic security cooperation) to being passive-aggressive (i.e. ignoring arms shipments). Not coincidentally, Turkey has had a similar relationship with Iran. And through all of this Turkey has remained a NATO member.
So... it's complex. The U.S. is perfectly capable of normalizing relationships with a country hostile to Israel, as long as that hostility isn't openly flouted. The degree to which Iran openly flouts its hostility to Israel, and to which the U.S. stylizes itself as Israel's protector, is context dependent and self-serving and really not about Israel, per se.
All of which is to say, the U.S. is antagonistic to Iran not because of Israel, but because of the U.S. And vice-versa. And it all quite clearly started with the Iranian Revolution.
[1] https://en.wikipedia.org/wiki/Israel%27s_role_in_the_Iran%E2...
I can see Iran being a regional threat, and whoever has power in that region controls a major portion of the world's energy supply. However, I don't see why an Iranian-dominated region is worse than a Saudi-dominated region. Iran seems more democratic and stable than Saudi Arabia, at a first approximation, but both are brutal, anti-democratic, oppressive regimes. And IIRC the Saudis have been the primary supporters of extremist Sunnis worldwide.
How is Saudi Arabia better for the U.S. than Iran?
Saudi Arabia, Egypt, and Jordan, on the other hand, all work closely with Israel.
They have money and influence. They have a stated and record of trying to harm the USA and the US allies. It's a threat and wouldn't take long to become a nuclear power (see pakistan).
What's hard to understand about the last 75 years? The speed of technology transfer is less than a change in Iranian regime.
Not implying Pakistan is a threat, SMH
