While the view that only states act as global actors is conventional, today significant diplomatic and cross-cultural activity is taking place in cities. Economic growth and fiscal experiments all occur in urban contexts. Cities are the center of the world economy, producing 85% of global GDP. Political reforms, social innovation, and protests and revolutions generate in cities. Criminal activities, terrorist actions, counterinsurgency, missile attacks (indeed, atomic bombs), and wars are centered in big cities. Pandemics spread in large urban conglomerates. Cities are sources of global pollution (80% of carbon emissions come from cities), as well as of environmental transformations such as urban gardening. Knowledge production, big data collection, and tech innovation all spur from intense interaction in cities. Cities are the meeting points between different cultures, religions, and identities. These increasingly international cities develop twinning networks and projects, share information, sign cooperation agreements, contribute to the drafting of national and international policies, provide development aid, promote assistance to refugees, and do territorial marketing through decentralized city-city or district-district cooperation. Cities do what ""municipalities"" used to do many centuries ago: they cooperate but also enter into intense competitive dynamics. To understand current sociopolitical dynamics on a planetary level, we need to have two mental maps in mind: the state-centered map and the nonstate centered map. With regards to diplomacy in particular, we must take into account the existence of a complex diplomatic regime based on different overlapping levels—the urban and the state.