Why has NATO taken so long in adapting its deterrence strategy to Russian revisionism and extending its military presence to the eastern allies? The setting up of NATO's Enhanced Forward Presence (eFP) in Poland and the Baltic states offers a critical case for examining the changing understandings of allied deterrence in the post-post-Cold War era. eFP is a story of negotiating the political acceptability and military credibility of NATO's modern extended deterrence strategy in the exposed eastern flank, and the navigation of the alliance security dilemma in relation to Russia while buttressing the eastern allies' physical and NATO's ontological security. This article traces NATO's extended conventional deterrence posture in the eastern flank from the adoption of the tripwire model shortly after Russia's annexation of Crimea to the commitment to defend ‘every inch of Allied territory’ via embracing the forward defence stance in 2022. Mapping the evolutionary curve of NATO's post-enlargement politics of deterrence through documentary analysis and interviews with diplomats and military representatives in NATO headquarters and national capitals, the article makes two contributions. Conceptually, it dissects the political rationalities and historical analogies underpinning contemporary allied deterrence strategy and posture in NATO's eastern frontline. Empirically, the study illustrates how allied deterrence is made to matter on the ground, and why this matters for deterrence credibility.