Most of Buchanan’s analogies are not apt. In this case the US is not just weighing morality against national interest, but it’s also making a choice between bad and worse. That is, between a murderous, retrograde tyrannical regime and a murderous, retrograde, tyrannical, expansionist rival. Buchanan seems to favor the latter. Israel probably has something to do with it.
It was Pat Buchanan who referred to Congress as “Israeli-occupied territory”.
Anyway, Saudi Arabia is by far the lesser of two evils if Iran is the other choice.