The ongoing Russia-Ukraine war remains the dominant barrier to NATO accession before 2027, as allies have avoided extending an invitation that would trigger Article 5 obligations amid active conflict with a nuclear power. Unanimous consent among all members is required, and recent summits have reaffirmed only an "irreversible path" without setting timelines or issuing formal offers. U.S. policy under the current administration and divisions among European allies further limit near-term momentum, consistent with the 96.5% trader probability against accession. Scenarios that could still shift odds include a swift negotiated ceasefire followed by rapid alliance consensus on reforms and security guarantees, though such developments would face tight procedural and geopolitical constraints within the remaining timeframe.