A security guarantee is a pledge by one country or alliance to provide military assistance to another if it is attacked. Security guarantees can take many forms — from the binding mutual defense clause in Article 5 of the NATO treaty to informal assurances. Ukraine gave up its nuclear weapons in 1994 in exchange for the Budapest Memorandum security assurances from the U.S., UK, and Russia — which Russia violated in 2014. Ukraine's insistence on hard security guarantees as part of any peace deal with Russia reflects its experience that assurances without enforcement mechanisms provide no real protection. The 2025-2026 peace talks have stalled partly on this issue.