This paper examines the impact on the welfare of resource-importing countries when they decide to form a customs union. We consider a dynamic tariff war amongst three players : one resource-exporting country and two resource-importing countries, with asymmetric demand. The tariff set by the importing countries and the (production?) tax set by the exporting country are time dependent . The resource is exhaustible. We consider two scenarios. In the first scenario, each resource-importing country chooses its own tariff rate non-cooperatively. In the second scenario, the two importing countries form a customs union, which requires them to charge a uniform tariff that maximizes their joint welfare . We first compare the countries' welfare under tariff war and under free trade and show that the gain or loss of welfare depends on the total size of each country and on difference between the sizes of the two countries. We then analyze the gain in welfare of forming a customs union as well as the distribution of the gains between the two asymmetric importing countries . We extend our analysis to allow for the exporting country to price discriminate between the two importing countries and determine the effect of price discrimination on the countries' welfare.