This is a randomized, active-controlled, open-label study of pembrolizumab given prior to surgery and pembrolizumab in combination with standard of care radiotherapy (with or without cisplatin), as post-surgical therapy in treatment naïve participants with newly diagnosed Stage III/IVA, resectable, locoregionally advanced, head and neck squamous cell carcinoma (LA-HNSCC). Efficacy outcomes will be stratified by programmed cell death ligand 1 (PD-L1) combined positive score (CPS) status. The primary hypothesis is that pembrolizumab given before surgery and after surgery in combination with radiotherapy (with or without cisplatin) improves event-free survival compared to radiotherapy (with or without cisplatin) given after surgery alone.