It has been 50 years since the Self-Defense Forces were created to protect the peace and independence of Japan and to deal with foreign acts of aggression. It is fitting, therefore, that the white paper on Japan's defense for this milestone year, released this week by the Defense Agency, takes up future roles for the SDF.

The SDF has evolved significantly over the past half century, reflecting changes in domestic and international situations. In particular, the 9/11 terrorist attacks on the United States have transformed the security environment in the post-Cold War world. That has posed a critical question for Japan: What kind of defense capability should the SDF develop?

The annual report specifically addresses this question on the basis of discussions conducted by an internal study group. It makes two key points: The first is that the SDF should improve its capability to deal with "new threats" to national security, such as weapons of mass destruction, missiles, guerrillas and international terrorists. The underlying belief here is that there is only a small possibility, if any, that Japan will be attacked in a conventional way involving large numbers of planes and ships.

The second point is that the SDF should take a more positive role in international peacekeeping activities. Japan has sent its troops on various noncombat missions abroad since 1992, when the Diet passed landmark legislation that opened the way for SDF participation in U.N. peacekeeping operations. These missions, the report says, have received "a full measure of public understanding and expectations and now represent a major type of the SDF's activities."

In other words, the SDF's roles have shifted in emphasis to meeting new security threats, rather than conventional military attacks, and to conducting international peacekeeping activities. It is true that this is accepted in principle by the Japanese public. At the same time, though, an assortment of problems present themselves when it comes to specifics.

The overriding need is to maintain coherence with the constitutional provisions and principles that renounce the use of force as means of settling international disputes. It is also essential to follow the established guidelines of defense policy, such as the principle of exclusive self-defense -- which says in effect that Japan will take defensive action only in the event of a direct attack.

The "new threats" may be more difficult to predict and, therefore, more difficult to deal with than the conventional ones, but this cannot be used as an excuse for scrapping the principle of exclusive defense or for allowing "preemptive" strikes. These possibilities are reportedly being considered by some in the government and the ruling parties.

The United States, it should be noted, was unable to prevent the 9/11 attacks despite its overwhelming military might, which has been demonstrated in Afghanistan and Iraq. It is unlikely that even the best-laid out missile defense plan will be able to detect or block sneak assaults such as a suicide bombing that uses a hijacked jetliner. It would simply be impossible to thwart every such strike by military force.

The lesson to be drawn is that military action has its limits. Instead of single-mindedly improving military responses to real or perceived threats, efforts must also be made to attack underlying problems, like poverty, and to stabilize situations and prevent conflicts through comprehensive programs including disarmament and confidence-building measures.

Caution is also required in expanding the SDF's international activities. The government believes that the existing legal framework for overseas SDF deployment -- consisting mainly of the PKO cooperation law as well as the ad hoc antiterrorism and the Iraq reconstruction support laws -- needs updating. To that end, it is preparing to establish permanent catch-all legislation.

Activities under the PKO law meet clear-cut conditions such as the existence of a ceasefire agreement between warring parties and the consent of the country involved. By contrast, support for U.S.-led military operations not explicitly authorized by the United Nations can raise the question of legitimacy. Moreover, deployment in a potential combat zone could expose SDF members to attacks from hostile elements or could lead to the use of force prohibited by the Constitution.

Therefore, severe conditions must be attached to SDF activities abroad. The last thing Japan should want is to expand the scope and nature of deployment in an unprincipled manner in the name of "international contributions" or "the Japan-U.S. alliance." To be sure, the SDF needs to be upgraded in line with reality, but, first and foremost, the SDF must be an organization that meets the basic tenets of the Constitution.