It's hardly irrational. You can argue the merits of their reasoning, but not that they have reasons. Don't dismiss viewpoints you don't agree with as irrational - that shuts down any attempt at a real dialogue or understanding.

A century ago the Middle East was sliced and diced with various powers getting part of the pie. Iran (Persia) was supposed to have been 'shared' between Britain and Russia, but the Russians had their own difficulties so the Tsar wasn't around to take on 'his share' of Iran. Nonetheless 'British Petroleum' got in there for a while.

It was Roosevelt that did the deal with the Saudis. The deal was as follows: sell us your oil in perpetuity and we will make sure that nothing threatens the 'royal' family.

Note the naming of the Saudi Aramco state oil business. 'Aramco' has the letters 'am' in it and that is because it is the 'Arab American Company'.

You are correct that the U.S. has 'irrational' opposition to Iran. This is because the U.S. has irrational opposition to any country that does not care about the dollar. The only truly sovereign nations are the ones that the U.S. has 'irrational' opposition to, everything else is a vassal state.

Believe it or not, US support has made KSA extremely weak, especially militarily. So its more like keep your enemies close, or in this case, oil targets. US could topple KSA in a matter of months.

Our policymakers have a rationale for that opposition though.

Now that rationale is probably foolish in the extreme.

But they do have a rationale.

Opposition to the Iranian regime is pretty straight forward. Which is a shame, because had history played out slightly differently, the huge, progressive middle class of Iran could have been in power and a natural ally of the US instead of the religious zealots.

And ummm, money...