When Iran violated international law by seizing Americans in the American embassy.
Iranians carrying garbage in an American flag, November 12, 1979:
https://www.granger.com/results.asp?image=0068113
Iran went downward from there.
Reagan should have stopped that shit long ago. Imagine the world if the mullahs were extinguished decades ago.
If I recall accurately, the Iranians were threatening to go after the Soviet embassy also and backed off when the Soviets essentially said they would rain hell on Tehran if their embassy and staff were seized.
I may be remembering that wrong. Been a long time.
It was at the time my belief that the US should have responded in that manner also. My disgust with Carter grew daily through the entire fiasco. And it has never lessened over the years.