Since all Department of Defense (DoD) missions depend on cyber assets and capabilities, a dynamic and accurate cyber dependency analysis is a critical component of mission assurance. Mission analysis aims to identify hosts and applications that are "mission critical" so they can be monitored, and resources preferentially allocated to mitigate risks. For missions limited in duration and scale (tactical missions), dependency analysis is possible to conceptualize in principle, although currently difficult to realize in practice. However, for missions of long duration and large scale (strategic missions), the situation is murkier. In particular, cyber researchers struggle to find technologies that will scale up to large numbers of hosts and applications, since a typical strategic DoD mission might expect to leverage a large enterprise network. In this position paper, we argue that the difficulty is fundamental: as the mission timescale becomes longer and longer, and the number of hosts associated with the mission becomes larger and larger, the mission encompasses the entire network, and mission defense becomes indistinguishable from classic network defense. Concepts generally associated with mission assurance, such as fight-through, are not well suited to these long timescales and large networks. This train of thought leads us to reconsider the concept of "scalability" as it applies to mission assurance, and suggest that a hierarchical abstraction approach be applied. Large-scale, long duration mission assurance may be treated as the interaction of many small-scale, short duration tactical missions.