The department of homeland security was created to coordinate efforts to

 Protect the United States from terrorist attacks.

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."

Note:

Another act passed after the 9-11 attacks, aimed at combating terrorism by whatever means deemed necessary for national security, was the PATRIOT Act.  The full name of the act was the Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001.

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