By building on a recent research comparing the EU and US and drawing on Riker''s influential theory of federalism, this books explores the origins of fiscal unions. It investigates early American history and traces its constitutional debates to argue that an internal threat - such as sovereign debt crisis leading to social unrest threatening the Union - triggers emergence of federal taxing powers - i.e. a federal fiscal union. It then contrasts the American experience of fiscal integration with the European one and subsequently concludes with the insights for the EU. It is a first monograph to compare the American and European models of fiscal integration, making two original contributions to the theoretical and empirical literature. In reference to the former, it introduces the concept of fiscalization, which defines the emergence of a ''fiscal union'' with federal tax powers. Concerning the latter, by analysing the Confederation period of the US and applying Riker''s theory using mainly unexplored primary sources, such as the protocols from state ratification conventions of 1787/88, this book adds to the US-EU comparative federalism literature. It shows that paradoxically, by not agreeing to give the EU fiscal capacity, so that they could protect their fiscal sovereignty, member states gave up more of this very fiscal sovereignty to the central institutions, than states in classical federations. This research allows the reader to learn about the similarities - and the differences - between the pre-Constitution US and the modern EU with regards to their fiscal arrangements; a comparison of the arguments that were used while debating those arrangements; and finally - the conditions under which central level of government in the systems of multi-level government is likely to get a power to tax.