"The United States is declaring that we will not use or threaten to use nuclear weapons against non-nuclear-weapons states that are party to the Nuclear Non-Proliferation Treaty and in compliance with their nuclear nonproliferation obligations,”
Seeking to set an example, the Obama administration said the US would consider use of nuclear weapons only in "extreme circumstances" and committed to not developing any new nuclear warheads.
But it said that while reducing the role of nuclear weapons in national security, the US would strengthen its conventional arsenal.
They don't like the idea of Washington and Moscow being on the front line. Better if the front line is on some distant farmland where the squaddies can slug it out.