The ultrafast nonlinear optical response of molecular ensembles is fundamentally altered under strong light-matter coupling. To rigorously isolate the genuine many-body contributions, an exact time-domain field-subtraction protocol is developed within a fully nonperturbative Maxwell-Liouville framework explicitly incorporating the two-exciton manifold in real space and time. This approach reveals that while collective cavity delocalization drives the macroscopic nonlinear signal toward a severe harmonic cancellation (an effect termed “spectral starvation”), intrinsic many-body molecular interactions robustly resurrect genuine polaritonic double-quantum coherences. This many-body resurrection is governed by a universal two-photon matching rule, ${\mathrm{\Delta }}_B+4J={\mathrm{\Omega }}_R$, linking molecular anharmonicity (${\mathrm{\Delta }}_B$) to the macroscopic Rabi splitting (${\mathrm{\Omega }}_R$) and excitonic coupling ($J$). Crucially, this resonance exploits the spatial mismatch between macroscopic polaritons and localized two-exciton pairs to break harmonic cancellation. For $J$ aggregates ($J<0$), this condition uniquely isolates the resonant many-body state below the dense manifold of localized dark states, protecting the macroscopic coherence from spatial fragmentation. This predictive framework establishes a direct phase diagram to engineer and protect optical nonlinearities across diverse strongly coupled platforms.

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• Light-matter interaction
• Nonlinear optical susceptibility
• Polaritons
• Optical microcavities
• Two-dimensional coherent spectroscopy