Japan’s new prime minister, Taro Aso, is well known — and not fondly remembered — by Japan’s neighbors as a pugnacious nationalist. As foreign minister from 2005 to 2007, Mr. Aso soured relations with China and South Korea and raised tensions throughout the region, praising the achievements of prewar Japanese colonialism, justifying wartime atrocities and portraying China as a dangerous military threat.

Now, the power brokers in the long-governing Liberal Democratic Party have made him Japan’s fourth prime minister in just two years and rebranded Mr. Aso as a “pragmatist.”

Mr. Aso is expected to focus on stimulating Japan’s stagnant economy. To successfully lead a 21st-century Japan, he will also need to swap nationalism for pragmatism when it comes to foreign relations. Japan’s future depends on cultivating stronger political and economic relations with China — its largest trading partner — South Korea and other rapidly advancing neighbors.

He has assured Washington that he will resist opposition efforts to shut down a Japanese naval refueling mission in the Indian Ocean — Japan’s risk-free demonstration of support for American and allied military efforts in Afghanistan.

What the United States most needs from Japan is a responsible strategic partner, not a government whose imperial reveries and symbolic muscle-flexing will provoke angry reactions across Asia.

Nationalism is enjoying a disturbing political revival because many Japanese fear that their country, once Asia’s clear economic leader, is losing ground to booming neighbors. The answer for that doesn’t lie in the nostalgic fantasies about Japan’s ugly past for which Mr. Aso has become well known.

Instead, Japan needs to modernize its economy by completing the market reforms begun by Junichiro Koizumi, the former prime minister. And it needs to modernize its foreign policy by treating its neighbors as equals. If Mr. Aso can be pragmatic enough to adopt that agenda, he is likely to be a successful prime minister.