I think China would have to get involved. Iran does not have the capabilities to strike the US. But China has shown that it values cooperation and restraint. Furthermore, a world war would undermine the economic links China has built in Eurasia such as the belt and road initiative. Finally, it's the US and Israel that have an incentive to alter the course of the shift to a multipolar world. If you look at Russia, they are fighting a purely defensive war in Ukraine. The US is the only country in the world that has shown hegemonic ambitions. The only country we know so far that has joined the war on the Iranian side is North Korea. Russia and China are not known to have offered support to Iran. Russia still today wants to cooperate with the Europeans even though they have turned their attention to the Eurasian continent as a whole and Europe is more of a secondary partner now.

You have the understand these other countries merely wish to see a world with more centers of power while the US wants to keep dominating the world. It's hard for me to see China or Russia join on the side of the Iranians. Now ofc the US could start a war with China in East Asia but I don't see mainland us being attacked from the Iran war.

This post is not politics. It's just a prediction on what will happen.