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 cm are not eligible, even if their total tumor size is 1.0 cm or greater). * Surgery for this lung cancer must be completed at least 28 days prior to registration. * Baseline CT chest must be performed within 28 days of randomization to ensure no evidence of disease. * Must have either previous NGS and PD-L1 results available using the Dako 22C3 antibody or have archival tissue of surgical specimen from current diagnosis available to perform analyses. PD-L1 results via the Dako 22C3 antibody will be performed per standard of care from a CLIA-accredited laboratory and are required for stratification. If NGS results are not available, subjects must be able to provide at least 10 x 10µm unstained and 1 x 4µm H\&E slides from current diagnosis for future NGS and/or other genetic analyses. * Demonstrate adequate organ function as defined in the protocol; all screening labs to be obtained within 28 days prior to registration. * Females of childbearing potential must have a negative urine or serum pregnancy test within 72 hours prior to registration. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. For subjects randomized to the pembrolizumab arm: If there is \> 72 hours between the screening test and C1D1, another pregnancy test (urine or serum) must be performed and must be negative before the subject may start C1D1. * NOTE: Females are considered of childbearing potential unless: they are postmenopausal; are surgically sterile; or they have a congenital or acquired condition that prevents childbearing. See Section 5.1.4 for definitions. * NOTE: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject. * A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies: * Not a woman of childbearing potential (WOCBP) OR * A WOCBP who is using a highly effective contraceptive method (failure rate of \<1% per year), or is abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) during the intervention period and for at least 120 days after the last dose of study drug. The investigator should evaluate the potential for contraceptive method failure (i.e., noncompliance, recently initiated) in relationship to the first dose of study drug. See contraceptive guidance in Section 5.1.4 of the protocol. * Participants who are HBsAg positive are eligible if they have received HBV anti-viral therapy for at least 4 weeks and have undetectable HBV viral load prior to randomization. Note: Participants should remain on anti-viral therapy throughout study intervention and follow local guidelines for HBV anti-viral therapy after completion of study intervention. Hepatitis B screening tests are not required unless: * Known history of HBV infection * As mandated by local health authority * Participants with a history of HCV infection are eligible if HCV viral load is undetectable at screening. Note: Participants must have completed curative anti-viral therapy at least 4 weeks prior to randomization. Hepatitis C screening tests are not required unless: * Known history of HCV infection * As mandated by local health authority * HIV-infected participants are eligible but must have well-controlled HIV on anti-retroviral therapy (ART), defined as: * Participants on ART must have a CD4+ T-cell count ≥350 cells/mm3 at screening. * Participants on ART must have achieved and maintained virologic suppression defined as confirmed HIV RNA level below 50 or the LLOQ (below the limit of detection) using the locally available assay at the time of screening and for at least 12 weeks before screening. * It is advised that participants must not have had any AIDS-defining opportunistic infections within the past 12 months before study entry (Day 1/randomization). * Participants on ART must have been on a stable regimen, without changes in drugs or dose modification, for at least 4 weeks before study entry (Day 1/randomization) and agree to continue ART throughout the study. * Note: HIV screening testing is not required unless mandated by local health authority. * As determined by the enrolling physician or protocol designee, ability of the subject to understand and comply with study procedures for the entire length of the study Exclusion Criteria: * Current lung cancer is \<1 cm or \> 4 cm i