In your scenario, the triggering event is if/when the US attacks a Chinese aligned territory. NATO is a defensive alliance. When the US attacked Iraq, NATO did

In your scenario, the triggering event is if/when the US attacks a Chinese aligned territory. NATO is a defensive alliance. When the US attacked Iraq, NATO did not follow. The only time NATO reacted was when Bin Laden attacked the US. Your scenario will only happen if China is the belligerent party.