This report presents results of one phase of a project called Future Combat Development Concepts. The project supports the Training and Doctrine Command's (TRADOC's) work toward understanding future combat development and doctrine issues. This particular report is intended to help TRADOC develop Army 21, a vision of the Army and an operational concept appropriate for 15 to 30 years in the future. The Army 21 effort entails the development of a set of plausible alternative worlds that could eventuate within that span of time. The results of this study are most relevant to those interested in future Army concepts and doctrine. Since Army 21 is an integral part of the Army Long-Range Planning System, the report will interest Army long-range planners. However, the methodologies used in this study are more general and should also be of interest to other long-range planners looking for an alternative to trend-based approaches to futures planning. Section 2 of this report describes the methodology the authors developed for Army 21 specifically and for long-range planning in general. It includes a description of the methodology and a comparison of that methodology with a more common approach to long-range planning that concentrates on trend extrapolation. Section 3 describes the implementation of the first step of the methodology: identification and analysis of the assumptions underlying AirLand Battle-Future (ALB-F). Section 4 describes the identification, by means of an innovative Delphi exercise, of plausible changes in the world in 25 to 30 years that would affect the Army and its operational concept. These steps are critical in the generation of alternative future scenarios that stress the ALB-F Umbrella Concept. Section 4 also explains how the assumptions and plausible changes are combined to generate the alternative future scenarios and describes the scenarios thus generated. Section 5 offers conclusions and observations. (5 tables).