Why Ukraine gave up its nuclear weapons — and what that means in an invasion by Russia

Three decades ago, the newly independent country of Ukraine was briefly the third-largest nuclear power in the world.

Thousands of nuclear arms had been left on Ukrainian soil by Moscow after the collapse of the Soviet Union in 1991. But in the years that followed, Ukraine made the decision to completely denuclearize.

In exchange, the U.S., the U.K. and Russia would guarantee Ukraine's security in a 1994 agreement known as the Budapest Memorandum.

Now, that