I think the lens of a localized, regional war in the Middle East is becoming too narrow. If we’re escalating things to the point of America’s direct and significant involvement (beyond shooting missiles out of the sky, but rather conducting its own attacks on Iran), then I think Iran’s potential allies extend beyond the region as well.
Most likely Russia first, as it’s already in a proxy war with the US/West.
Potentially China, but not likely until it’s most advantageous for them to do so. Or perhaps they’ll enter opportunistically, such as attacking Taiwan if America’s Naval might is sent to the Gulf, opening a potential three front war.
The economic connection between these three has continued to grow in recent years, and may be reason enough.
Perhaps not directly, but if America steps up it’s involvement, they will be relying on their relationships with allies in the region to do so.