Ukraine surrendered its nuclear weapons and decommissioned it's intercontinental bombers in exchange for security guarantees from the west, it was called the Budapest memorandum. The guarantees were even reaffirmed in 2009 however when Ukraine was invaded and asked for help, nobody answered.
Ted Cruz actually explained it to Trump during one of the presidential debates: "Ukraine used to be the third-largest nuclear country on the face of the Earth. Ukraine voluntarily gave up its nuclear weapons because the United States of America came in and said if you hand over those nuclear weapons, we will ensure your territorial integrity from Russia. We made a commitment, and then the Obama administration has broken its word."