W Bush focusing on fighting insurgencies while China was rising, led to the US not having the military industrial base and stockpiles of certain munitions. If the US uses too many munitions fighting Iran, it will be greatly weakened for a fight against China. This also weakens US deterrence against China.
However, if Iran is in too much chaos to pump oil, or if it is friendly enough with the US that in the event of a China-US war it would not sell oil to China, that would be a huge advantage in starving China of oil in the event of a war.