Before you open the national border, you have to start small!

First, get everyone not to lock their front door. Then, ban all fences and "no trespassing" signs.

If that pilot program is successful, then think about opening the borders.

It occurs to me that any country that opens its borders will be instantly flooded with every murderous scumbag that is fleeing from the law elsewhere. You will also be an instant transfer point for the trade of weapons, narcotics and so on. Not to mention human trafficking.

I guess how you deal with that is that you open your border to foreign law enforcement and give them a carte blanche about to conduct their investigation how they see fit, and make arrests.

Speaking of arrests, open borders means that someone can sneak into your country, kidnap someone, and sneak out.