ABSTRACT Military suicide rates increased 61% from 2008 – 2019 and rates have increased faster in the U.S. Air Force (USAF) compared to other branches. Currently, the predominant military suicide prevention approach is to try to remediate suicide risk after suicidal individuals are identified. No RCT-validated universal programs shown to reduce vulnerability to suicide are in wide use. To fill this gap, the Wingman-Connect Program is a group-based intervention that strengthens protective relationship networks and skills for managing career and personal challenges, to reduce vulnerability to suicide across the broad USAF population. This proposed Hybrid Effectiveness-Implementation trial tests the effectiveness of the Wingman-Connect Program on individual suicide risk and on base-level suicide attempts. We will examine theory-driven mediators and moderators and implementation of the program as delivered by US Air Force (not research) personnel under real world conditions across 2 sequential early career phases. This effectiveness study is the critical next stage in the translational pipeline toward large-scale roll-out to prevent suicide deaths. To rigorously test effectiveness, we co-developed with USAF partners a 2-stage randomized design. (1) The first stage of randomization will be at Initial Technical Training, in which 396 classes of USAF personnel will be randomized to Wingman-Connect or to an active control (N=2,970 Airmen) and followed for one year. These classes send a high proportion of graduates to 8 operational bases. (2) The second stage of randomization will occur among these 8 operational bases, which will be randomized in pairs to start implementing WC at 4- month intervals (stepped wedge design). Once WC has been initiated at each base, all entering first-term Airmen will receive WC, with ~17,400 total Airmen trained across all bases. This 2-stage design will yield robust, multi-level effectiveness findings. Aim 1: Test effectiveness of WC on reducing (a) self-reported suicide risk and (b) base-level rates of suicide attempts. We will evaluate (a) individual level outcomes of suicide risk, depression, and occupational problems up to 1 yr; and (b) base-level administrative records of suicide attempts. Aim 2: Evaluate theory-proposed network health mediators and moderators. WC is expected to increase Airmen's positive social bonds, group cohesion, morale, and healthy coping norms in their social networks; those changes will contribute to reduced suicide risk, depression and occupational problems. Aim 3: Examine implementation determinants and mechanisms, and refine Implementation Package. Key implementation outcomes will be USAF implementers' fidelity delivering WC (n=24-30) and engagement in training/technical support. Implementer fidelity and engagement is expected to be predicted by base implementation climate and WC embeddedness into base communications and support activities.