As various scholars have noted, a “new world order” of global governance is emerging that involves a wider and more decentralised cast of decision-makers focused on an ever-widening array of transnational problems, such as climate change, global migration, health pandemics, and sustainable development, among others (e.g., Slaughter, 2005). In this new global order, national governments and state-based international organisations are viewed as inefficient, unequipped to deal with existing transnational challenges, captive to elites and, in some cases, simply dysfunctional (Barber, 2014). Nationstates (and the international bodies that represent them) are finding that their independence, sovereignty and borders – the traditional virtues of statehood – are barriers to the types of cooperation required to solve the cross-border global problems we face today. State-on-state “gridlock”, as well as the partisan paralysis that prevents many national governments and state-based international organisations from accomplishing their agendas, risk a more profound “sovereignty default”, which can result in a failure to act or to effectively govern at the international level. This has created an opening for subnational actors, such as city governments and civil society organisations, to fill the gaps where the state has failed to act, and thereby become agents of international policymaking and problem-solving (Barber, 2017).