Abstract Research Summary This paper examines how incumbent firms' market positions and interdependencies across their submarkets influence their responses to entry threats. We adapt a model of capacity deterrence to show that because premium and low‐cost incumbents face different demand functions and operating costs, they experience different tradeoffs between ignoring, deterring, and accommodating threatened entry. In addition, the interdependencies within and between a premium incumbent's submarkets influence its responses. Using data on incumbent responses to entry threats from Southwest Airlines between 2003 and 2012, we find that (a) full‐service incumbents expanded capacity while low‐cost incumbents did not respond significantly, and (b) full‐service incumbents expanded capacity less aggressively in submarkets that had less substitutable customer segments and submarkets that were more complementary with their unthreatened submarkets. Managerial Summary An immutable market position is a core competitive advantage. Using data on incumbent responses to entry threats from Southwest Airlines between 2003 and 2012, we find that (a) full‐service (FSC) incumbents expanded capacity while low‐cost (LCC) incumbents did not respond significantly, and (b) FSCs expanded capacity less aggressively on routes that were expected to have a large number of business passengers and routes that connected to their international hubs. These results suggest two sources of positional immutability: While one set of past choices (e.g., those about submarket substitutability or complementarity) provide a barrier against imitation, another set of past choices (e.g., those about products and costs) generate incentives for a tough defense, both deterring entry by firms from a different position.