New START represents a commitment that both countries have honored for decades to not deploy even more nuclear weapons. It was signed in 2010 to replace the 1991 START I Treaty, which helped reduce the total number of nuclear weapons to around one-fifth of the number that existed at the worst moments of the Cold War.

Make no mistake – the U.S. and Russia still possess the vast majority of the world’s nuclear weapons, and the nukes that still sit ready to be used can still destroy the world as we know it. But preserving the limits set by the treaty is essential because every nuclear weapon, and every new weapons system dreamed up in the absence of legal limits, is an opportunity for a mistake, a rash decision, a moment of overconfidence with unthinkable consequences.