TOKYO  Japan’s decision to intervene in currency markets to weaken the yen and shore up its export-driven economy could be only the first step in a long battle  one made more difficult because Japan, unable to find support among its trading partners, must go it alone.

Japan moved Wednesday to prop up the United States dollar and weaken the yen, directly manipulating its currency for the first time since 2004. Japanese monetary authorities appeared to have bought dollars and sold yen. That move started a wider market rally that sent the dollar up by about 2.55 percent for the day, to 85.62 yen.

The maneuver by Japan’s finance ministry came after the yen had reached 15-year highs in recent weeks, elevated by its status as a haven among risk-averse global investors. The strong yen has hampered Japan’s recovery from last year’s severe recession by weighing on the competitiveness of its exports, which account for a bulk of the nation’s economic growth.

“We conducted a currency intervention to check excessive volatility in currency markets,” Yoshihiko Noda, the finance minister, said Wednesday. “We will continue to watch currency market moves and take decisive steps if necessary, including intervention.”