To date, the shortest wavelength of an ultraviolet current-injection nitride laser has been limited to ∼340 nm. This begs the question of whether there is a fundamental limitation that restricts the realization of injection lasers below this wavelength. This letter investigates this issue. We find that, apart from being the decisive factor for the crystalline quality of nitride epitaxial layers, residual strain plays yet another crucial role in modifying the band structure of (Al,Ga)N quantum wells. A critical analysis of quantum-confinement effects, strain, and the polarization properties of strained (Al,Ga)N layers is essential for the realization of deep ultraviolet laser diodes.