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We study the thermodynamic stability of dilute C, N, O, B, and C+N interstitial distributions in bcc iron, combining parameter-free density functional theory (DFT) in the generalized gradient approximation and microscopic elasticity theory. This scheme allows us to fully capture the long-range elastic impurity-impurity interactions using moderately sized DFT calculations. Employing this approach we compute temperature-concentration phase diagrams including the effects of external pressure, and provide direct insight into the formation mechanisms of martensite. For all investigated impurities, except for B, tetragonal states are predicted to be preferred even at low impurity concentrations. The preference is shown to originate from a thermodynamically driven orientational ordering of the interstitials.