The Most Important Critical Mission to Protect Homeland

Background:
The original homeland security strategy for the United States came in the aftermath of the events of 9/11. Even though there was supposed chatter of potential terrorist attacks, later revelations over the years have determined that elements of the Bush administration ignored warnings, including some that were far more detailed than previously revealed.
The first strategy was just a beginning to develop and institute bold and necessary steps to protect the United States from further terrorist attacks. Since this time, the United States government has developed and published other strategies: 2006 National Strategy for Combatting Terrorism; 2007 National Strategy for Homeland Security; 2018 National Strategy for Counterterrorism to name just a few. With the change of political climates, presidential administrations, and current threat analysis against the United States, new and shifting strategies will come into play.
Question:
The last United States National Strategy for Homeland Security was published in 2002, that is almost 18 years old. Much has changed over the years and some challenges remain the same. The strategy’s objectives of homeland security in order of priority are to: prevent terrorist attacks within the United States; reduce America’s vulnerability to terrorism; and minimize the damage and recover from attacks that do occur. Furthermore, they articulate six critical mission areas to accomplish this formidable task. Identify which critical mission area is the most important to protect the homeland and why?