By Kana Inagaki and MinJung Kim

Japanese Prime Minister Shinzo Abe’s economic plan seemed to sail off almost flawlessly this year as stocks soared and the yen tumbled from levels late last year. Investors world-wide were excited that this might be Japan’s long-awaited revival after 15 years of deflation and decline.

But has Japan really turned a corner?

The ride is suddenly bumpier as volatility jolts the markets, sending bond yields and the yen higher and the Nikkei Stock Average into a bear market. Skeptics question Mr. Abe’s resolve to deliver radical growth strategies, while gyrations in asset prices have raised doubts about the Bank of Japan’s handle on markets.