They can't get through eastern Ukraine. What world war? They have no means (and the mediocre means they had before the beginning of the war in Ukraine, have been slashed in half, to say nothing of vast degradation of still quasi-operating hardware), they'd get stomped by NATO within a month and throw the nuclear card on the table to end it.
Iran? Trivial to contain within their own borders, and we'd ignite an armed revolution by flowing massive amounts of weapons into Iran.
Belarus? Barely has a military worth mentioning in this context.
And there's good reason to be skeptical of China joining Russia in a world war.