We should be wary of over-diagnosing the decline of American power
lol. the spanish civil war example is counter-factual to non-interventionist thinking.
like Ukraine, it was quasi "fascists" on the right, and a dysfunctional alliance of liberals and leftists in opposition to the fascists.
the liberal-left alliance was in power, and was overthrown by the "right", which was the pattern for something like 200 years.
the spanish "fascists" (actually ultra conservatives pretending sometimes to be "fascist") were supported by Hitler and actual fascists from Italy. Formally, the "liberal" UK and USA didn't intervene to help the liberal-left alliance, presumably because isolationism was dominant in the USA, and the USA had carried out an international war against Spain in 1899, and won. So, not amigos.
In any case, the intervention by the Nazis helped the "right" (Franco) to win. at a minimum, Nazi support it did not hurt the Spanish right.
the spanish left was fragmented and one specific hard line communist faction got Soviet support (from Stalin?)
the result of that was that, as Orwell observed (as a war participant), sometimes other communists and anarchists were more worried about getting killed because by the pro-Stalin communists in vicious leftist infighting than when fighting Franco's right wing forces.
in any case, the dysfunction and infighting on the left was a major reason that the left lost the war, even with Soviet support.
Orwell or some other historian I read made the case that the Soviets didn't actually want a successful leftist state in Western Europe because it would have created competition internationally with the USSR's dominant position as a communist state. In that analysis it was to the USSR's benefit to keep western europeans fighting among themselves in any way possible.
Making things worse is part of the plan. It's important to come to terms with that.