Eastern enlargement constitutes a puzzle for the rationalist analysis of international institutions. The problem is not the interest of the CEECs in joining the Western organizations. Here rationalist institutionalism provided a plausible explanation in both the NATO and the EU cases. The puzzle is on the supply side. According to the security approach, enlargement was not necessary for an unthreatend and unrivalled NATO. In the EU case, it may have been useful for an economic organization in rivalry with the United States (and Japan); but then it was not efficient. This lack of efficiency also causes the power approach to fail – Eastern enlargement dilutes rather than strengthens the power of the Western organizations. Moreover, the auxiliary condition of preclusion proved unconvincing with regard to both the timing and the scope of Eastern enlargement.

Finally, even the welfare approach failed, although it is the least restrictive of the rationalist approaches and therefore the most likely one to be confirmed. First, the CEECs are poorer and located in a less stable environment than Western Europe. In addition to transaction and autonomy costs, enlargement was therefore expected to produce crowding costs that would exceed the CEECs' contribution to the club goods. NATO enlargement increases security risks for the old members and causes extra costs not balanced by the military or financial contributions of the new members.