Adjuvant Pembrolizumab vs Observation Following Curative Resection for Stage I Non-small Cell Lung Cancer (NSCLC) With Primary Tumors Between 1-4 cm

Inclusion Criteria: * The participant (or legally acceptable representative if applicable) must provide written informed consent for the study. The participant may also provide consent for future unspecified research samples. However, the participant may participate in the study without participating in the future unspecified research sample collection. NOTE: Initial informed consent will remain valid throughout the 12-week period between surgical resection and study registration unless, in the opinion of the treating investigator, the participant experiences a significant change in medical or mental status. * Males and females age ≥ 18 years at the time of consent. * ECOG Performance Status of 0-1 within 28 days prior to registration. * Patients must have undergone complete surgical resection of their stage I NSCLC between 4-12 weeks prior to registration and have negative surgical margins (R0). * NOTE: Both squamous and non-squamous histologies are allowed into the study. Cancers with a histology of "adenosquamous" are considered a type of adenocarcinoma and thus "non-squamous histology". * NOTE: Staging will be according to the AJCC 8th edition. * Pathological tumor size must be 1.0 - 4.0 cm in greatest dimension. NOTE: According to AJCC 8th edition, subjects with lepidic predominant adenocarcinoma should be staged based on their invasive tumor size and not their total tumor size (i.e., subjects with lepidic predominant tumors whose invasive tumor size is less than 1…