"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." [0]

We can see this security guarantee in action right now. Same will happen with NATO's mutual defense guarantee.

[0] https://www.npr.org/2022/02/21/1082124528/ukraine-russia-put...