NATO’s Article 5 collective-defense clause has been invoked only once in its history, following the 9/11 attacks, establishing a high evidentiary and consensus threshold that requires an armed attack on alliance territory and unanimous agreement among members. Traders price the “No” outcome at 85 percent largely because the Russia-Ukraine conflict, while intense, has remained contained to non-NATO territory, with allies providing weapons and training but avoiding direct combat that would cross the Article 5 threshold. Russia has escalated gray-zone operations against NATO states, yet these have not produced the clear armed attack needed to trigger the clause. Ongoing diplomatic efforts around potential Ukraine security arrangements and U.S. pressure on European allies to assume greater conventional defense roles by 2027 have further reinforced expectations that the alliance will continue managing risks short of formal invocation through the end of 2026.