What is the fine-grained topic of the following text: In 2004 David's son Tim (the one with the cool gun in the center), joined the Marines, where he is an MP attached to HMX1 and Marine One, an elite group of MPs that act as personal guard for the President of the United States which he is standing next to below. Only return the topic.
Tim joining the Marines and serving as a personal guard for the President of the United States.