When an ally provides diplomatic cover for restraint

Can one country really stop another sovereign power from acting?

Usually not if the state is determined; an ally's restraint often supplies diplomatic cover for a decision the state already prefers.

A powerful sovereign state will usually act if it has genuinely decided to do so. An ally cannot simply veto that choice. The ally can instead provide a public explanation for restraint, allowing leaders to say they yielded to friendly counsel rather than lacked resolve. This face-saving diplomacy lets a government avoid using power without publicly admitting that it never intended to act.