In the 1990s, NATO began a course of enlargement and transformation to remain relevant in Europe's post Cold War security environment. As part of its commitment to enlargement, it admitted three new members--Poland, Hungary, and the Czech Republic in 1999 and has plans to admit more countries in the future. NATO's enlargement has profound military implications for the U. S. and its allies in terms of future planning and shaping strategies. Both have been driven primarily by political imperatives, not by a sense of direct threat, but by an environment-shaping agenda of democratization and integration. This report develops and applies an analytical framework for thinking about the determinants of future NATO enlargement, the specific defense challenges they pose, and shaping policies that might aid in addressing these challenges.