Both countries are already in the midst of ambitious and extremely costly efforts to “modernize” their nuclear arsenals. Of all the weapons now being developed, the two generating the most anxiety in terms of that nuclear threshold are a new Russian ground-launched cruise missile (GLCM) and an advanced U.S. air-launched cruise missile (ALCM). Unlike ballistic missiles, which exit the Earth’s atmosphere before returning to strike their targets, such cruise missiles remain within the atmosphere throughout their flight.