We know nothing stopped the US in the end. It's kind of the rub of the problem of a unipolar world with a single Superpower. (The multipolar has its own challenges, but they are different.)Did that stop it though?
Why do you ask?
They did plenty.
Fx do you think Russia should get props for threatening to veto the US proposal to make its invasion a UN mandate, while France which did the exact same thing, should not? Possibly instead be expected to directly have use force to try to stop the US, i.e. its main strategic ALLY, up to and including military. It's not as is Russia or China breathed even a noice about being prepared to try that either.
US allies confronting the US and denying didn't just demonstrate their sovereignty – they also took a certain risk in directly opposing the wished of their most powerful strategic ally, who was taking them for granted. There are tons of double standards involved over the 2003 Iraq war.The French and Russians did the exact same things against the US, but the French got the US hate and ridicule over it ("Cheese eating surrender monkeys"; "Freedom fries"; "French victories; did you mean French defeats?" Etc.)
Do you think US allies were more morally obliged to downright fight the US directly than everyone NOT a US ally over this?
A more reasonable question for you to ask would actually be how come so many of the US European allies OPPOSED it on this.