Although great attention has been paid to space weapons in relation to the Strategic Defense Initiative (SDI), there seems to be little recognition that platforms performing an SDI role could establish a space blockade. The United States must recognize the feasibility of such a blockade and take steps to offset it. Possible steps include depending on arms control pacts to keep weapons out of space, developing terrestrial systems for use in breaking a blockade, or putting weapons in space before an adversary can establish a blockade. Problems in verifying the nature of orbiting vehicles reduce the reliability of arms control. Advantages space weapons will have over surface-based systems argue against dependence on restorative measures undertaken from the ground. Thus, putting weapons in space is an option that should be explored. There are, of course, arguments against this option. Perhaps the most cogent is that using space for military ends may be destabilizing. Merely placing US weapons in low orbit, even if they were deployed solely for SDI, would threaten rivals because of the possibility of a US blockade. Moreover, such platforms would be vulnerable if another power put weapons in orbit. However, a mix of low-orbit SDI forces and high-orbit reserve forces should be less vulnerable and less threatening. Such a defense could be improved by creating a high-orbit keep-out zone for each space power.