Researchers have found that even international merger and acquisition (M&A) are popular, they usually fail for its complexity. A large number of studies looked on the failure from financial, non-financial, managerial and HRM perspectives, but reasons for the failure is not yet understood. Recently few studies have tried with process perspective to penetrate into merger integration without much success. We argue that focus on pre- and post-merger stages is good but not enough to provide a total picture of the process. This study uses a longitudinal approach to analyze the establishment, development and consolidation of a cross-border merger between banks from four Nordic countries. Based on theories on strategic motives and culture, a theoretical framework has been developed showing how merger can lead to better competitiveness and performance. By using a qualitative method, data has been collected through semi-structured interviews with officials from the merging firms, and secondary materials and documents during the period of 1997-2015. The merger process is described in four stages. The first stage deals with the initiation and establishment of the merger. Critical period and cultural clashes are covered in the next stage. The third stage concentrates on the development of synergy and other measures. The final stage reflects on how the process has consolidated and what result it has so far achieved.