Western operational doctrine has long classified the conduct of war across four domains — land, sea, air, and space — and added cyberspace as the fifth domain following the 2010 NATO Lisbon Summit. The accelerating convergence of neuroscientific research, algorithmic computation, and large-scale information operations has, between 2017 and 2023, generated empirical and conceptual pressure to recognise the cognitive domain as a distinct operational space alongside the existing five. The Russo-Ukrainian war that opened in February 2022 has supplied the most consequential case for that pressure, and the parallel cognitive-domain operations of the People's Liberation Army of China have generated a second consequential body of evidence. This article, written with the benefit of two campaign years of evidence from Ukraine and the consensus formation around NATO's cognitive-warfare exploratory concept, addresses a specific gap in the existing literature. Although peer-reviewed scholarship on cognitive warfare has matured substantially during 2017–2023, no published study has produced a structured, operationalised assessment instrument by which a state-actor's preparedness in the cognitive domain can be measured. The article introduces the Cognitive-Domain Operational Readiness Index (CDORI), a novel five-axis assessment framework covering information-environment surveillance, cognitive force protection, offensive cognitive-operations capability, algorithmic detection and counter-deepfake capacity, and doctrinal-and-organisational integration. Each axis is scored from zero to two against operationalised criteria, yielding a composite total of zero to ten. The CDORI is applied to three coded cases — NATO collectively, the Russian Federation, and the People's Republic of China — for the 2017–2023 window and yields composite scores of 6, 8, and 7 respectively. Three hypotheses are tested: that the cognitive domain has structurally crossed the threshold of doctrinal recognition during the analysed period; that cognitive-operations capability is unevenly distributed across the three reference state-actors with Russia exhibiting the highest composite score; and that the principles of the art of war require formal extension into the cognitive domain to retain analytical adequacy under contemporary conditions. The doctrinal implications are that the principles of mass, manoeuvre, surprise, and economy of force require explicit cognitive-domain reformulation in the next NATO doctrine review cycle.