A down-to-earth briefing on the Strategic Defense Initiative from a team of New York Times correspondents who won a Pulitzer Prize for a 1985 series on the controversial project. Expanding on their earlier coverage, the authors trace the origins of SDI (unveiled by President Reagan in March of 1983) back to the mid-1960's, when work began on a prototype ABM (antiballistic missile) system. Advances in electron beam, kinetic-energy, laser, and allied technologies distinguish Star Wars from prior efforts to defend against long-range rocketry. But whether Buck Rogers hardware can be harnessed to provide a worthwhile measure of protection, much less realize the Chief Executive's stated goal of rendering nuclear weapons ""impotent and obsolete,"" remains a very open question. The authors make clear, moreover, that political considerations rival substantive technical problems (notably, computer programming) when it comes to development and deployment decisions. By way of example, Star Wars research has introduced new elements into both deterrence doctrine and disarmament negotiations, in part by undermining such longtime articles of faith as MAD (mutual assured destruction). Also unresolved in an aerospace conflict whose principal protagonists are the US and USSR are the roles of ""lesser nuclear powers."" In the meantime, many scientists, political leaders, and even military analysts have grave misgivings about SDI's obvious offensive potential. Weighing the program's pros as well as cons, the authors offer a balanced if somewhat inconclusive and decidedly cautionary overview. Their meticulously documented audit cites authoritative sources ranging from staunch supporters like Edward Teller (so-called father of the H-bomb) through less sanguine observers, including the American Physical Society, which recently released a detailed critique of the breakthrough-dependent SDI project. In brief, then, a thoughtful and sobering contribution to a vital debate on national priorities. The thoroughly accessible text has photographs and line drawings (not seen).