And the practicalities of a war would necessitate an effort to stop that, to prevent the spread of enemy combatants behind the front line. Even if you search everybody passing through the border for weapons.. ...it's America. They can get weapons here.
The inevitable result is a massive human rights disaster, that makes the present status quo look like paradise.
It's would actually be really easy to completely close the border if there was the political will for it. The US spends way more money on frivolous things every year than it would take to fully militarily fortify the entire border.
The inevitable result is a massive human rights disaster, that makes the present status quo look like paradise.