Changing technology means that national and global security can no longer be separated. One of the gravest threats to national security, according to a report from the Pentagon, is global climate change, with extreme weather events provoking migrations, humanitarian crises, and impacting military preparedness. Moreover, climate change can affect food and water security. Climate change can only be tackled on a global level.
Next, many weapons of mass destruction, even if deployed locally can have global effects. A nuclear war or release of epidemic diseases as part of biological warfare anywhere in the world could have devastating global effects. That means that the United States can not assume that North America would be safe if a major war broke out in Africa or Asia. Also, weapon systems can now reach anywhere in the world in a matter of under an hour, meaning that no place is safe from attacks from rogue states.
Finally, security now includes not just physical matters but cyber security. Because the internet is global and allows almost instantaneous transmission of malicious code, threats such as computer viruses and hacking are global as well as national issues.