The Social Democratic Party (SDP) made a natural and logical decision on Sunday and broke away from the ruling coalition of the Democratic Party of Japan (DPJ) and the People's New Party.

The SDP decided to do so because its leader, Mizuho Fukushima, was dismissed from her Cabinet post as consumer affairs minister after refusing to sign the government's plan to relocate the US Marine Corps Air Station Futenma in Okinawa Prefecture.

Like the SDP, Prime Minister Yukio Hatoyama, too, had pledged to seek the relocation of the air base "at least, out of the prefecture". In announcing her party's departure from the ruling camp, Fukushima said: "In practicing politics, we want to take responsibility for what we said." Hatoyama should ruminate on her words.

The SDP's policies on key issues of national security, military bases and the Constitution define the core of its political platform. The SDP has apparently learned a lesson from its past mistake.

In 1994, then Socialist chief Tomiichi Murayama became prime minister of a government supported by the party's coalition with the Liberal Democratic Party (LDP) and New Party Sakigake. As Murayama took office, the SDP changed its positions on key security issues and acknowledged the constitutionality of the Self-Defense Forces and promised to maintain Japan's security alliance with the US.

The abrupt policy shift set the stage for the SDP's long and steady decline, which has continued to this day. The SDP has been deeply traumatized by the experience.

In forming the ruling coalition, the three parties had agreed on 34 policy positions for 10 key issues. The Japan-US security alliance is only one of them. If, however, it is a policy on which the SDP cannot compromise, the party has no choice but to leave the coalition.

But the SDP also decided to keep working with the Hatoyama government on the rest of the coalition agreement. The party says it will not reject cooperation with Hatoyama's DPJ for Upper House election in summer, either. These decisions were made, it says, because a majority of Japanese wish to prevent a revival of the old politics. We welcome the party's wise choice.

Cooperation among political parties doesn't have to be dictated by the logic of either alliance or confrontation. Experiences show that there can be various forms of political cooperation among parties.

The SDP's action plan for 1997 proposed cooperation with the LDP government led by the then prime minister, Ryutaro Hashimoto, from outside the Cabinet.

It was a looser form of alliance focused on cooperation on specific policies rather than the ordinary ruling coalition, which involved participation in the Cabinet.

Japan's political parties are yet to master the art of policy cooperation. They are not good at working out agreements with other parties on specific policy proposals and revising related bills as needed through talks based on various forms of cooperation.

The question now is whether the SDP's decision will lead to the formation of new rules for such flexible cooperation.

It is the DPJ that is facing the need to change its way of thinking. The party has taken many strong actions in its efforts to push its key policy initiatives, including a radical reform of the nation's postal service, through the Diet or the parliament. If it continues using its huge majority in the Lower House to get bills passed by forced votes, it cannot hope to hold constructive policy talks with other parties.

The SDP's departure has reduced the ruling coalition's majority in the Upper House to a razor-thin margin. If it loses its control of the Upper House in the upcoming election amid evaporating public support, the DPJ may start political maneuverings to regain a majority.

If possible opposition control of the Upper House leads to a return to politics focused on grabbing power through whatever means necessary, as symbolized by the aborted agreement between the LDP and the DPJ on a "grand coalition", there can be no progress in the nation's democracy.

Establishing transparent and fair rules for cooperation among parties would radically change the ways the government and the Diet operate.