Under the Imperial Household Law, which governs most matters of protocol related to Japan’s monarchy, Masako was not even permitted to attend the sacred ascension ceremony for her husband this month. Women are not allowed to sit on the throne.

“The Japanese public is so excited and it seems like they are very much expecting Masako to take advantage of her previous career as a diplomat,” said Kumiko Nemoto, a professor of sociology at Kyoto University of Foreign Studies. “I’m sure she understands that there are lots of forces she needs to take account of and not overstep certain boundaries.”

Because the Imperial Household Agency, which manages the imperial family’s every move, is one of the most traditional institutions in Japan and is dominated by mostly older Japanese men, “they expect Masako the empress to behave with always a certain smile and a very subservient type of womanhood,” Ms. Nemoto said.

Monday’s banquet was not Masako’s first time sitting next to an American president. Just a month after her wedding in 1993, the public swooned when she sat between President Bill Clinton and President Boris N. Yeltsin of Russia at a state dinner in Tokyo and conversed easily with both in their own languages.

But before long, the Japanese news media became consumed by one question: when Masako would have a child.

Even after her daughter, Princess Aiko, was born in 2001, the Imperial Household Agency prohibited Masako from traveling abroad in case that would interfere with her ability to get pregnant again, prompting her to sink into depression and withdraw from public appearances altogether.