What is the fine-grained topic of the following text: The article presents information on the relations of South Korea with the U.S. Based on the 1953 Mutual Defense Treaty between the two countries, the U.S. will help South Korea defend itself against external aggression, which is why the U.S. has maintained military personnel in the country. Only return the topic.
U.S.-South Korea Relations and the 1953 Mutual Defense Treaty