We have been following the Monroe Doctrine since its inception. In the 1800s, the threat to us was from foreign powers using South and North America as a base to attack us from. Since then, we have made it progressively harder for foreign powers to do that by first winning the Mexican-American War, then purchasing Alaska and ultimately at the end of the 19th century by annexing Hawaii.
However, technology has made the world smaller and the Atlantic and Pacific oceans are not the barriers they used to be - as was perhaps best demonstrated when the Japanese attacked Pearl Harbor.
As a result, because technology has made the world smaller, the geography covered by the thinking behind the Monroe Doctrine has progressively gotten bigger.
Taking a bite out Hawaii is a far cry from taking the continental US.
And as history demonstrated, America's power in retribution is formidable. That, without more (as long as it is recalled and credible), is a deterrent.
Which countries, in your mind, are credible threats to the 50 US states?
Mind you, I am all for a strong defense. And, as you appear to be a student of military, infantry is what it takes to hold a country. Not that I want a war on US soil, but what country in its right mind would mount a war against heavily armed and angry citizens?
“However, technology has made the world smaller and the Atlantic and Pacific oceans are not the barriers they used to be - as was perhaps best demonstrated when the Japanese attacked Pearl Harbor.”
This cuts both ways. We don’t need to maintain foreign bases manned with hundreds of thousands of personnel to strike anywhere in the world. Carrier groups can keep us in range of any of our potential enemies. ICBMs and B2s give us striking range from our own territories to anywhere on the planet. Drones piloted from Edwards can hunt down terrorists anywhere, and can be logistically supported from very small Naval ships.
Boots on the ground is expensive knee-jerk political correctness, pretending populations that support terrorists will “like” us if we meet them on a level playing field mano a mano rather than bomb them into the stone age from a distance. We need to treat terrorists as representing their native countries, give their governments a chance to deal with their “criminals” themselves or face our full wrath as though the country itself had attacked us.