What is the fine-grained topic of the following text: President Barack Obama's visit to Hiroshima in Japan will be the first ever by a sitting US president, remembering the hundreds of thousands of lives lost and scarred when the United States became the first and (thus far) only power to use nuclear weapons, dropping two in August 1945 to force Japan's surrender, ending World War II. Only return the topic.
President Barack Obama's visit to Hiroshima, Japan