Under conditions of high capital mobility, states are pressurised into various forms of tax competition to attract or retain capital and investors. When this occurs, the capacity of domestic institutions autonomously to generate fiscal policies is constrained. What exactly, if anything, is unjust about this phenomenon? This paper argues that tax competition puts particular pressure on internationalists, who must acknowledge that its occurrence makes our obligations of global justice more demanding, and that such obligations require supranational institutions in order to be discharged. However, to the extent that tax competition is unjust from an internationalist point of view, this is due to the specific harm it does to states. Thus, the paper makes three contributions to the literature: it shows that (1) tax competition has an impact on the demandingness of internationalist obligations of justice; (2) discharging such obligations commits internationalists to accept a higher level of supranational institutionalization than is currently acknowledged; yet (3) both the specific content of such obligations, and the kind of supranational regulation that is needed to discharge them, differ from those which cosmopolitans support. In sum, there are some cosmopolitan bullets which internationalists must bite, and others which they should continue to resist.