The core argument of this paper is that for both Russia, NATO and many of the Baltic littoral states, evolving antagonism and mutual suspicion is likely to draw them into a growing pattern of rivalry in the Baltic sea. This is in part, because escalation on land, including so called hybrid threats, would quickly reach untenable thresholds. Moreover, for NATO the Baltics lacks strategic depth to be viable and therefore require maritime depth, especially to give NATO options to avoid escalation towards threats of employment of tactical nuclear weapons. This focus on the importance of the maritime is somewhat ironic, given that the Baltic states themselves are focused on immediate territorial forward defence, protection from internal subversion, or exotic threats in cyberspace. Yet the sea and the wider maritime Baltic space is as pivotal for defence contingency planning by all parties, Russia, NATO and individual states. To explore these issues, this paper draws on NATO historical doctrines on similar defence-in-depth dilemmas during the later Cold War, and on operational planning insights as suggested by recent Russian and NATO military exercises. In conclusion, at multiple levels of analyses, the Baltic sea is likely to witness both increased strategic salience, as long as the wider relationship between Russian and the west remains so tense.