This paper proposes a sequential merger formation game to study how trade policy can influence firms' choice between intra-national and cross-border mergers in an international Cournot oligopoly with a cost structure à la Perry and Porter [Perry, M. and Porter, R.H., 1985. Oligopoly and the Incentive for Horizontal Merger. American Economic Review 75(1), 219–227.]. We find that the equilibrium market structure depends heavily on: (i) the level of trade costs; and (ii) whether or not active antitrust authorities are incorporated within the sequential merger game. In addition, it is shown that whenever mergers occur in equilibrium, they occur in waves and the merger wave comprises at least one cross-border merger.