The U.S. says it needs smaller nuclear weapons as a deterrence against other so-called "Great Powers."
The Pentagon's newly updated Nuclear Posture Review outlined two new types of nuclear weapons to be produced: low-yield nuclear options for ballistic missiles, which could be launched from a submarine, and nuclear-armed submarine-launched cruise missiles.
The theory goes Russia expects the U.S. wouldn't use a high-yield nuclear weapon if it detonated a smaller, low-yield nuke. Defense officials say having those smaller nuclear weapons is now a necessary deterrence.
The submarine-launch capability would also make the weapons effective against North Korea and could be a reassurance to Japan and South Korea.
The Pentagon says it will focus on developing low-yield ballistic missile warheads in the short term and develop nuclear-tipped submarine-launched cruise missiles in the long term.