What is New START and why is Russia 'suspending' its role in the treaty?

Russian President Vladimir Putin said Tuesday that Moscow is "suspending" its participation in New START, the only remaining nuclear arms control treaty between the United States and Russia.

"Our relations have degraded, and that's completely and utterly the U.S.'s fault," Putin said during his annual state of the nation address. He added: "If the U.S. conducts tests, then so will we. Nobody should have any illusions that global strategic parity can be destroyed."