We are increasingly living in a world of forever wars, wherein neither party has a foreseeable determinate pathway to victory. This chapter explores three challenges for traditional just war theory raised by forever wars. First, I discuss and reject the claim that forever wars necessarily fail the proportionality and reasonable prospect of success conditions of jus ad bellum. Second, although forever wars may not be disproportionate, they do suffer from compounding errors and indeterminacy in assessing likely future costs and benefits. Finally, I consider time-variant value and discounting, wherein future goods are deemed to possess less value than present goods. Discounting is a feature of the appraisal of financial and monetary goods, and it seems to play a role in some moral judgements also. Subjecting the expected costs and benefits of war to discounting over time significantly impacts the moral permissibility of forever wars. Time variability impacts ad bellum judgements about the justification of war as well as in bello decisions, generating a reason to prefer weapons that generate immediate strategic advantage, but whose collateral costs often occur far in the future.