Remember when searching that you are looking for specific elements or facets of national security. You will likely have better search results if you use those terms (i.e. cybersecurity, climate change, domestic terrorism, etc.) You may also find that adding the term "security" will help narrow your search, and you may see some search improvement by also using United States. For example:
"climate change" AND security AND "United States"
will bring back more focused results than just searching for "climate change."