It depends on the share of external loss that falls outside the deciding jurisdiction. Three regimes: (1) Local externality (water pollution, regional smog, land-use change) — all external parties reside in the deciding jurisdiction. Per-country action is optimal; no leakage problem. (2) Trade-coupled externality (deforestation behind cattle exports, embedded water) — harm happens in country B, demand pull comes from country A. The brake belongs at the consumption point; border adjustments like the EU's CBAM are the correct instrument. (3) Global externality (greenhouse gases, ozone depletion, ocean fishery collapse, gain-of-function pathogens) — the externality is genuinely fungible across geography. Coordination is constitutive of the brake: below ~50% of global activity in the coordinated bloc, leakage is significant; above ~70% it is de minimis.