IN one sense, there's nothing very new about Japan's new leadership. Prime Minister Kiichi Miyazawa came up through the ranks of the dominant Liberal Democratic Party (LDP); he's a veteran of its factional politics and knows exactly what the various power centers within the party will demand of him.For example, Mr. Miyazawa may be inclined to smooth over the trade disputes that led the agenda during talks with US Secretary of State James Baker this week. Washington would like Japan to lift its barriers against rice imports. But Miyazawa has powerful LDP colleagues who remain bound to the rice farmers' lobby; they'll resist any attempt to yield on that emotional issue. Such inflexibility, however, runs counter to today's global political currents. The end of the cold war is reshaping policy in Tokyo as elsewhere in the world. For 40 years, Japan has concentrated on developing itself economically while US forces maintained a protective presence both on its shores and in the region. The US military umbrella is likely to fold, at least partially. This needn't imply that Japan will have to bolster its military - and thus alarm its neighbors. The country's adherence to peace for the last four decades, while economically convenient, has been genuine. It learned from the militaristic past. The fundamental issue now is what role an activist Japan, freed from subordination to the US, should play in building a more peaceful world. That task begins close to home, in a region that still remembers the horrors of World War II. Tokyo dispenses more foreign aid than any other capital, much of it within Asia. Commercial and industrial development has been a primary aim. A greater commitment to humanitarian aid could help dispel doubts about Japanese benevolence. More direct Japanese involvement in resolving regional conflicts like Cambodia would be helpful too. Japan was criticized during the Gulf war for standing on the sidelines and offering cash. But international activism can take forms other than jumping into battle. Miyazawa is known as an internationalist. In the past, he has had useful ideas on how to reduce third-world debt, for instance. He's also thought to be interested in taking a prominent part in next year's world environmental summit in Brazil. Contrary to US wishes, Japan may push for a vigorous response to the threat of global warming. Japan faces a world still concerned about its past militarism and suspicious that its motives are solely commercial. The new prime minister has an opportunity to prove those worries wrong.