If we build a wall to keep Mexicans out, the Mexicans will return the favor and stop selling us stuff or at the very least make us pay a lot more for their products like tequila.
If we torture people we suspect to be terrorists or bad guys, they will torture back, and the next time a soldier is kidnapped in one of those countries such as Afghanistan or Syria, they will pay for it. Will Trump make the trip to Dover to greet the body parts?
If we threaten China with economic reprisals for their trade practices, they will have no reason to not continue counterfeiting iPods and using slave labor to make tennis shoes and Trump ties. Oh, and they also hold a lot of our debt. They might want to call them in.
If we rattle our sabre at North Korea, they might just shoot back. Their leader is not too stable, either.
This is reality.