I analyse the negotiation between two countries, or regions, that are trying to make an agreement in order to internalize externalities. Local preferences are local information, but reluctance to participate in the agreement is signaled by delay. Conditions are derived for when it is efficient to restrict the attention to policies that are uniform across regions - with and without side payments - and when it is optimal to forbid side payments in the negotiations. While policy differentiation and side payments let the policy be tailed to local conditions, they create conflicts between the regions and thus delay. If political centralization implies uniformity, as is frequently assumed in the federalism literature, the results describe when centralization outperforms decentralized cooperation. But the results also provide a foundation for this uniformity assumption and characterize when it is likely to hold.