The literature shows publics are generally reluctant to raise defence spending in peacetime, yet attitudes in frontline states remain underexplored. Evidence is also scarce on how allied force-posture signals shape mass opinion. We study Lithuania under heightened threat after Russia’s 2022 invasion of Ukraine and Germany’s decision to station a brigade (in Lithuania). Moving beyond Western European samples and U.S.-centric scenarios, we field a large, preregistered online survey experiment (N=3,200; April 2026) that varies credible, country-specific posture cues: a U.S. drawdown, a German increase, their combination, and a status-quo control. The outcomes are support for national defense spending, willingness to defend, and policy trade-offs. Embedding theory-driven mediators (readiness, fairness, loss salience, and entrapment risk) allows us to distinguish free riding from reciprocity and to assess asymmetries in how losses and gains are processed. The study contributes micro-level evidence on burden-sharing preferences in an exposed ally and improves external validity through realistic vignettes tied to actual deployments. We will share preliminary findings at the July 2026 conference.