The United States has fought alone before, and it is possible that it will do so again. However, the far greater likelihood is that the United States will fight with an ally or as part of a coalition-another Desert Storm is more likely than another Grenada. Thus, the ability of U.S. forces to operate effectively with those of other nations is an important issue. Unfortunately, doctrinal and technological changes under way in the U.S. Army may be making it more difficult for it to operate in conjunction with its friends and allies. This documented briefing offers an approach to determining how impending changes-specifically those associated with Force XXI-could affect the Army's ability to operate with allies or as part of a coalition. It does so by identifying the aspects of Army XXI that could create difficulties in multinational force compatibility and then recommending policies and procedures to ameliorate the problems. The analysis focuses on the Army's ability to operate with NATO forces, excluding the three most recent members. The rationale underpinning this focus is that these forces pose the best case. They have some of the most modern equipment of all our allies and partners, share doctrinal similarities with U.S. forces, and have operated in conjunction with them for years. If Force XXI advances create compatibility problems with NATO forces, the problems are likely to be far greater with other allies and partners. An analysis of Army XXI shows that while it is an evolutionary rather than revolutionary advance, some of its key aspects are indeed likely to make it more difficult to operate as part of a multinational force. Problems crop up in the areas of C4I (command, control, communications, computers, and intelligence), force employment, and logistics. The problems become most severe in short-warning scenarios that require NATO countries to project their power beyond Europe.