BBC News Online asked Japan analyst David Powers to explain.
What is the Self Defence Force?
The simplest way to describe the Japanese Self Defence Forces (there are three of them - Ground, Air and Maritime) is as the Japanese army, air force and navy. The only problem is that ever since the American-led Allied occupation of Japan rewrote the constitution just after the Second World War, Japan has legally been forbidden the right to maintain such forces. After the atrocities perpetrated by the Imperial Army, the idea was to prevent Japan ever becoming a threat to its neighbours again.
But almost as soon as the ink was dry on the new constitution, the Cold War broke out; and the Americans realized their good intentions were rather impractical - particularly with so many potential Communist enemies right on Japan's doorstep in the Soviet Union, North Korea and China. So Japan was given the right to defend itself, and the Self Defence Forces were set up in 1954, two years after the Allied occupation came to an end.
What does it do?
For most of the post-war period, the Japanese have taken the name of the Self Defence Force very literally. It has been there to defend Japan - nothing else. In fact, it took another 40 years before the main opposition party would even recognize the existence of the Self Defence Force as constitutional.
Even so, it is one of the most modern and best-equipped forces in the world. It has state of the art technology, and trains regularly with the United States. But in keeping with its purely defensive role, it does not possess offensive weapons, such as intercontinental missiles or bombers. Nor does it have nuclear weapons, although Japan is known to have the technology to build them.
The Self Defence Force plays an important role in air-sea rescue in the seas around Japan. However, the concept of civilian control over its operations is so tight that there was a damaging delay before the Self Defence Force was mobilized to help rescue victims of the massive Kobe earthquake in 1995. Troops waited on the sidelines for vital hours until the prime minister gave the go ahead for them to be used.
What about UN peacekeeping operations?
Although Japan has played a very active role in the United Nations, particularly as one of the most frequently elected non-permanent members of the Security Council; it's been almost conspicuous by its absence from peacekeeping operations. During the Gulf War in 1991, Japan came under heavy international criticism for sending money, rather than manpower.
At the time, there was little Japan could do - the constitution specifically prohibits the country from using force as a means of settling international disputes. Nevertheless, it prompted a vigorous debate about the role it should play in international security. After much acrimony, it was eventually decided to allow the Self Defence Force to take part in UN operations, but under very strict limitations.
Japanese troops are not allowed to take part in any combat. The only weapons they can carry are small arms, and they can use them only if their own lives are in danger. In practice, this means they cannot be deployed in areas where fighting is likely to break out. So, although Japan has participated in UN operations in Cambodia, the Golan Heights and East Timor, its role has been extremely limited.
What changes have been made?
Three specific measures involving the Self Defence Force are:
None of these changes involve Japan in actual fighting, but they have proved controversial all the same.
The government had to tread carefully to avoid upsetting strong pacifist sentiments at home and fears among Japan's neighbours that militarism might be on the rise again. The first steps were taken a couple of years ago, when Japan revised its defence guidelines to pledge logistical support for United States troops in case of an emergency in an unspecified region around Japan.
Prime Minister Junichiro Koizumi came out in firm support of the United States very quickly after the attacks in New York and Washington in September; and wants Japanese help to be clearly visible.
Why should there be any controversy if no fighting is involved?
Many people see it as the thin end of the wedge - a way for Japan to start flexing its military muscles again. Although there's some domestic opposition, it's much smaller than on previous occasions. Japanese people say that's because they realize after the Gulf War that they need to stand side by side with the rest of the democratic world when it comes under threat. And they point out there is no plan to remove the constitutional ban on going to war.