I think this is the best answer. Shia and Sunni also celebrate (sometimes different) holidays on slightly different dates, and the Shia version of the Shahada includes (an apparently optional?) reference to Ali.

I want to say that the Iranian revolution is also important here, but I'm really not sure, because revolutionary Iran has worked with the USA in the past (in the Iraq War, for instance). The process of the West turning against the Shia has probably been gradual. Saddam was also Sunni and is apparently thought of as a hero in the Sunni world. There are so many people who belong to each of these sects, but my general impression in talking with them is that Sunnis are more westernized (less likely to wear the hijab, see Fanon's fascinating take on the versatility of the hijab in the Algerian struggle) while Shias are more hostile to western influences, but there are many exceptions. Hamas is currently the only major Sunni organization I can think of that is fighting genocide, while Hezbollah, Ansarallah, and the Iranian government are Shia. I follow a Sunni news group on telegram that will sometimes post the strangest shit about Syria (they claim that Iran and Hezbollah have committed atrocities there and are convinced that Jolani is actually independent from western influences). They will also say that Hamas is better at fighting than Hezbollah, basically because Hamas films a lot of their combat while Hezbollah is more shy to do so.

The vast majority of the world's Muslims are Sunni, but the two groups seem to contend with each other a lot in the Middle East only (plenty of Shia also live in India and Pakistan). Although there are huge numbers of Muslims in places like Indonesia, India, and Africa, it sometimes feels like only a few countries really exist in the Muslim world—Iran, Saudi Arabia, Egypt, Syria, Palestine, the UAE. Algeria for instance is remarkably low-key even though it's right next to all the action, it has fifty million people, a growing economy, an anti-imperialist stance, many connections with Russia and China, and a huge military (its concern largely seems to be with French influence over Algerian gas exports and the endless conflict with Morocco / France over Western Sahara). Before the Iranian revolution, the ideological battle in the Middle East was between Nasser's secularism and the religious fervor of Saudi Arabia; now the battle is between Sunni and Shia thanks to the rupture of the Iraq War.

I think Hegel's notion that everything is a lot more closely connected than we might realize is instructive here.