Answer:
Essentially, the Department of Defense deals with mostly foreign countries and military actions against the US homeland. For example, the Department of Defense will upkeep equipment and facilities, as well as maintaining a force at home.
The Department of Homeland Security, on the other hand, deals primarily with domestic emergencies and counterterrorism. For example, the Department of Homeland Security would access funds to secure America's borders, as well as gather information on potential terrorists and keep tabs on them. They may also provide emergency relief funds and supplies plus personnel's to areas affected by natural disasters.
Learn more about the differences between the two Departments, here:
https://brainly.com/question/4323382 - See adriansp's answer.