Reliable guarantees do not come on paper, as the protector can always opt out. The most effective pledge in history, by the United States to Cold War Europe, did not rest on the NATO treaty, which contains no automatic obligation. The real commitment was embodied in 300,000 US troops plus their nuclear weapons on the firing line. These kept the peace because they spelled out to the Soviet Union that an attack on the allies was an attack on America itself. This lesson holds for the future, as well. The guarantor must tie his hands, and he does so with maximal credibility by putting his own forces in harm’s way.