A nuclear conflict has the potential to end life as we know it. Though progress has been made in reducing the number of missiles in both the US and Russia, both countries still have many thousands of nuclear warheads.
There have been numerous close calls in the past due to simple monitoring malfunctions. One of the dangers of a war with Syria is that it could escalate into a nuclear war. Furthermore, it's possible that a terrorist group could set off one or more stockpiles of nuclear warheads. After all, a small group of nonviolent protesters got deep into the secured area of a nuclear facility without really trying.
As the country with the most warheads, and a history of actually using them, it is our responsibility to be a leader in reducing the number of warheads.