Why is the work of department of homeland security so complex?

The work of the Department of Homeland Security is complex because it involves numerous different agencies and is aimed at protecting against a wide variety of threats to the American homeland.

Context/details:

The Homeland Security Act, which created the Department of Homeland Security, was passed in 2002, following the September 11, 2001 attacks by terrorists against the United States, which involved hijacking planes and flying them into the World Trade Center and the Pentagon.  The Department of Homeland Security describes its mission as ensuring "a homeland that is safe, secure, and resilient against terrorism and other hazards."

The Homeland Security Act of 2002 (Title I, Section 101), listed the top three priorities of the Department of Homeland Security as:

Also listed as part of Homeland Security's mission is that it will  "monitor connections between illegal drug trafficking and terrorism, coordinate efforts to sever such  connections, and otherwise contribute to efforts to interdict  illegal drug trafficking."

The Department of Homeland Security also acts "as a focal point  regarding natural and manmade crises and emergency  planning," and works to coordinate efforts between various agencies and law enforcement agencies  in order to achieve its security goals.  According to the Department of Homeland Security's own website, the Department of Homeland Security combines "22 different federal departments and agencies into a unified, integrated Cabinet agency."

map scale refers to the relationship (or ratio) between distance on a map and the corresponding distance on the ground. for example, on a 1: 100000 scale map, 1cm on the map equals 1km on the ground.