Japan's August 1999 agreement to engage in ballistic missile defense cooperation with theUnited States has the potential for contributing materially to the ability of the U.S. Navy to field anAsian regional defense against intermediate-range ballistic missiles, a goal that has long receivedstrong support from Congress. Although Japan has committed to research and development (R&D)cooperation on four elements of a Navy interceptor missile, Tokyo has not made a decision toacquire a missile defense capability. It is even less clear how far Japan might be prepared to movein the direction of an integrated regional missile defense cooperation arrangement. The extent ofJapan's future participation in missile defense will be governed by a number of considerations,including its threat perceptions, overall national defense strategy, regional relationships,constitutional constraints, domestic political impact, technical feasibility, and cost. The relativeimportance of these factors cannot be established with any precision -- any one or combination ofthem could have a make or break effect on Japanese decisionmaking. To date, these considerationshave had a mixed and sometimes contradictory effect on Japanese policy. This report documents and analyzes Japanese perspectives on ballistic missile defense andon participation in the U.S. missile defense R&D program, with particular attention to current trendsin Japanese security thinking, major actors in the policymaking process, and political andconstitutional constraints. It notes areas of convergence as well as issues on which American andJapanese perspectives tend to diverge. Finally, the report briefly addresses a number of policyconsiderations for Congress and the Bush Administration in light of ongoing uncertainties aboutJapan's participation. For broader background on U.S.-Japan relations and security cooperation, seeIssue Brief IB97004, Japan-U.S. Relations: Issues for Congress (regularly updated). Since the mid-1990s, Congress has supported the development of a missile defense capability to protect forward-deployed U.S. forces in the Asia-Pacific area, regional allies, and Taiwan fromshort- and medium-range missiles, a goal that requires some level of Japanese support--if onlyhosting U.S. missile defense forces. The 1991 Persian Gulf War highlighted the threat of short-rangeScud ballistic missiles and the inadequacy of the Army's Patriot missile defense system to protectU.S. ground forces and facilities. Similar concerns have been expressed regarding the U.S. Navy'scurrent lack of a defense against both short- and intermediate-, or "theater"-range, ballistic missiles,and cruise missiles. Testimony by numerous defense and intelligence officials highlighted thegrowing threat posed by the development of intermediate-range ballistic missiles capable of carryingweapons of mass destruction (WMD) by anti-U.S. regimes ranging such as North Korea and Iraq. Congressional Reaction to China's 1996 Missile "Tests" in theDirection of Taiwan. Following China's firing of ballistic missiles in the vicinityof Taiwan during a Taiwan Strait confrontation in early 1996, Congress acted to support thedevelopment and deployment of a missile defense system explicitly oriented towards Asia and thewestern Pacific. Section 1533 of the FY1999 National Defense Authorization Act ( P.L. 105-261 ,signed into law on October 17, 1998) required the Secretary of Defense to "carry out a study of thearchitecture requirements for the establishment and operation of a theater ballistic missile defensesystem in the Asia-Pacific region that would have the capability to protect key regional allies of theUnited States," and to submit a report to Congress not later than January 1, 1999. The report wasto describe any U.S. missile defense system either currently deployed or being developed "that couldbe transferred to key allies of the United States in the Asia-Pacific region to provide for theirself-defense against limited ballistic missile attacks." It was to be submitted in both classified andunclassified versions. Congress clarified the term "key regional allies" in the conference report( H.Rept. 105-736 ), identifying these as Japan, South Korea, and Taiwan. (1) The Department of Defense (DoD) delivered a 15-page unclassified version of thecongressionally mandated Theater Missile Defense (TMD) report in May 1999. The report focusedon five ballistic missile defense systems currently under development for U.S. forces, and describedoptions for the defense of South Korea, Japan, and Taiwan against an attack by fewer than fivemissiles of under 3,500 km (2,170) range. The report assumed that the missiles would not employspecial measures to evade destruction, such as the use of decoys or altered trajectories. Theunclassified version of the DoD report addressed hypothetical architectures for each country'ssituation, but did not attempt to suggest or describe any region-wide system architecture, nor did itaddress the most challenging types of threats. (2) Changing Context of Congressional Support for Missile Defensein the 107th Congress. Congress continued to show support for developing anddeploying a "theater" level missile defense capability in 2001, but also for more ambitiousdevelopment objectives that might allow TMD systems--especially the Navy's sea-based TMDcapability to serve as a basis for an early national missile defense capability. (3) For instance, on March 28,2001, Rep. Vitter introduced two related bills expressing strong support for an Asian missile defensecapability but also for upgrading the planned speed of the Navy Theater-Wide (NTW) interceptormissile to give it the ability to intercept North Korea's Taepo Dong I missile and Iran's Shahab 5missile, and requiring the Department of Defense to conduct at least one test against an incomingmissile with the flight characteristics, including velocity, of the Taepo Dong I. (4) Rep. Vitter also introduceda companion bill, the Defense Against Regional Threats Act of 2001, Sec. 2 of which would makeit U.S. policy "to provide for deployment as soon as is technically possible of effective missiledefense systems capable of defending Israel, Japan, the Republic of Korea, Taiwan, and all membernations of the North Atlantic Treaty Organization against ballistic missile attack." Sec. 3 of the billwould make it U.S. policy "to seek continued negotiated burdensharing agreements with the nationsspecified in section 2 to share the costs of development and deployment of ballistic missile defensesystems." Both H.R. 1282/1283 appeared to reflected impatience on the part of a number of Membersof Congress at the determination of the Clinton Administration to avoid testing or deploying missiledefense systems that would violate the ABM treaty. Although neither bill went beyond referral tothe Armed Services Committee, the proposed legislation implicitly supported the decision of theBush Administration to radically revamp the U.S. BMD program, with the goal of applying variousABM technologies across a range of missions, including the early deployment of a capability todefend U.S. territory against limited attacks by intercontinental-range ballistic missiles that mightbe launched by "rogue" states. The United States military uses the term Ballistic Missile Defense (BMD) as a genericdesignation for systems designed to defend against ballistic missiles of whatever range--fromshort-range "Scud" type missiles to intercontinental ballistic missiles (ICBMs). Thus, both TMDand National Missile Defense (NMD) systems are BMD systems. TMD systems are intended to bedeployed in a military theater of operations to defend against short-range and theater-range (up to3,500 km) ballistic missiles; NMD systems are intended to defend U.S. national territory againstcontinent-spanning missiles, i.e., ICBMs. Currently, both the U.S. Army and the U.S. Navy aredeveloping anti-missile systems for theater-wide defense, but the U.S. Defense Department haschosen the former Navy Theater Wide (NTW) anti-ballistic missile system, now designatedSea-Based Midcourse Defense (SMD), as the most appropriate system for an "Asian" TMD. In a series of policy statements beginning with a speech by the President at the NationalDefense University on May 1, 2001, the Bush Administration indicated an intention to enlarge andredirect current BMD programs in a way that tends to erase the clear distinction between TMD andNMD. The reasoning behind this decision appears to be at least two-fold. First, the relevanttechnologies are applicable across the whole range of BMD threats. Second, and relatedly, certainprograms currently in development for lower tier threats are deemed to have the potential, if suitablyenhanced, of serving as a stop-gap, near-term NMD capability in the absence of a full-scope NMDsystem. This reasoning applies particularly to the former Navy Theater Wide (NTW) program, whichhas been the focus of U.S.-Japan TMD cooperation. One possible sea-based option would buildupon the technologies being developed in the former NTW program to develop a system that couldbe deployed on the Navy's Aegis cruisers stationed off the U.S. Pacific coast, with the mission of intercepting ICBMs in mid-course, outside the atmosphere. Another concept is to deploy a sea-basedsystem in the Sea of Japan with a capability to intercept North Korean intercontinental missiles intheir assent, or "boost" phase, when they are most vulnerable. (5) A more technologicallyambitious concept under active consideration involves deploying aircraft with laser systems capableof destroying missiles in their boost phase. This report discusses and analyses the Administration's approach to missile defense and itsimplications for U.S.-Japan cooperation on missile defense, but the report's point of departure is thetraditional delineation of types of anti-missile systems based on the characteristics of the specificballistic missile threats that they seek to counter. In other words, the main ballistic missile threat toU.S.