The current border is simply a dotted line drawn on a map. Less than 2% of the border has fencing or other physical barriers that prevent illegal entry from Mexico. It is true that all an intruder must do to enter the United States currently is to walk across an unprotected area in any of the border states (California, Arizona, New Mexico and Texas). No need to buy and airline ticket, or board a boat or submarine. Just walk across. That's it.
The same is true in China. Just walk across. There's no fence there, either. But there is one big, BIG difference: in the People's Republic of China, it is a crime to enter their country, and those who do so without permission from the Chinese government do so at great risk to their lives, if not their freedom.
China has borders between themselves and Russia, North Korea, Pakistan, India, Nepal, Bhutan, Burma, Laos and Vietnam. Thousands and thousands of miles. Thousands and thousands. And along those thousands of miles of border there is not a foot of fencing, zilch, zero, nothing to prevent anyone from walking right into China. Nothing except the fear of breaking Chinese law, and ending up disappearing without a trace, or winding up in a Chinese prison for years if not decades (if not life).
Nobody enters China. Nobody. Or at least nobody enters China without permission. And if they do, they pay dearly.
What's the difference? China enforces its borders with stiff penalties, including death. The U.S. lays down the welcome mat, and provides all comers (read: invaders) with social security benefits, health care benefits, free public education, access to public transportation systems, public parks, and free public (enter the freebie of your choice here).
And we think its impossible to secure our borders? Nothing to it. Position a few machine gun turrets here and there, detect an invader or two and gun them down, and eliminate their threat to our border security. It wouldn't be long before our border is rock, solid secure. Without fences.
Piece o' cake. Works like a charm in China. It would work here, too.