TOKYO — Japanese Prime Minister Fumio Kishida, who shepherded the U.S.-Japan alliance to its strongest point and ramped up his country’s defense spending in an increasingly volatile Asia-Pacific region, has announced he is to step down as leader of his party and, therefore, the country.

       During his three years as Japan’s prime minister, Kishida shone on the diplomatic stage, becoming a leading voice in Asia against Russia’s invasion of Ukraine, warning it could embolden China to attack Taiwan and spark a regional war.