We propose a simple model of the international monetary system. We study the world supply and demand for reserve assets denominated in different currencies under a variety of scenarios: under a Hegemon vs. a multi-polar world; when reserve assets are abundant vs. scarce; under a gold exchange standard vs. a floating rate system; away from or at the zero lower bound (ZLB). We rationalize the Triffin dilemma which posits the fundamental instability of the system, the common prediction regarding the natural and beneficial emergence of a multi-polar world, the Nurkse warning that a multi-polar world is more unstable than a Hegemon world, and the Keynesian argument that a scarcity of reserve assets under a gold exchange standard or at the ZLB is recessive. We show that competition among few countries in the issuance of reserve assets can have perverse effects on the total supply of reserve assets. Our analysis is both positive and normative.