Forget the gains of the U.S. markets. Japan's Nikkei (NIKKEIINDICES:^NI225) blew away the competition this week, posting an eye-popping gain of 6.3% over the past five days. What helped spur Japan's markets? Prime Minister Shinzo Abe's inflationary tactics are finally paying off: The yen sank to the important 100 yen-to-the-dollar mark this week, part of a 14% year-to-date fall against the U.S. currency. Other nations won't sit by idly as Japan drives down its currency, however; let's look at the latest from across the Pacific.

Conflict around the falling yen
The ever-weakening yen is a boon for Japan's exports, which have had to deal with a strong currency for years and a stagnant economy for two decades. Abe's 2% inflation target is still a long way off in Japan's current deflationary climate, but the yen's fall is a sign that things are headed the right way.

Japan will need the boost: If Abe's aggressive moves can't jump-start its flat economy, the nation in coming years will run into a two-tiered gauntlet of an aging population -- the result of a low birthrate in the recent past -- and a giant public debt that's more than double the country's GDP. Those two events, each massive in scope, although probably years away from becoming major problems, could derail Japan's economic hopes.

Yet Japan's gains have sparked concerns among other economic leaders around the globe. China has already spoken out against continued aggressive moves from Japan's central bank, saying that its Asian rival should stop its attempts to devalue its currency -- even as China looks to keep its yuan low against foreign currencies itself. This week at a meeting of the G7, U.S. Treasury Secretary Jack Lew joined the chorus of voices, saying that Japan's economic stimulus attempts needed to stay within international guidelines to avoid a potential currency war.

Currency devaluation could be a boon for top Japanese exporters, yet Nissan (OTC:NSANY), as one of Japan's top automakers, hasn't been able to take advantage this year. The stock has risen more than 9% year to date -- something most investors can't complain about -- but that's far behind the Nikkei's double-digit surge in 2013.

The company's full-year earnings report this week inspired no one on Wall Street. Nissan's net profit pushed higher by just 0.3% for the past 12 months ending in March, and like fellow Japanese automaker Toyota (NYSE: TM), Nissan took a hit from the Chinese market. The company does expect net income to jump by 23% for the current fiscal year, but with Chinese and European sales slumping, Nissan will have to rely on American sales and revenue from Japan to fuel any rise. The weak yen should help overseas, but Nissan underperformed its Japanese rivals this past year -- a problem that a falling currency won't solve.

Nissan's done better this year than electronics maker Advantest (UNKNOWN:ATE.DL), but this stock absolutely blew up over the past week. Advantest's shares shot higher by more than 9%, wiping out pessimism over the company's weak earnings released a few weeks ago. Advantest's net loss and operating profit both fell below its guidance, and despite this week's investor optimism, the future's murky for this company. Financial site TheStreet downgraded the stock last week, citing Advantest's falling earnings, among other issues.

It was anything but a good week for Japanese telecom carrier NTT DoCoMo (NYSE:DCM), however. The wireless leader saw its stock fall 1.4% over the past five days even after reports emerged that Sony (NYSE: SNE) could be releasing a new phone, the Xperia A, under DoCoMo's network. An FCC filing has shown the Xperia A with a 5-inch screen, and DoCoMo investors could know more when the company unveils its mobile offerings for the summer on May 15.