Come on now, this is ridiculous and such slogans are to be seen all over the netword today.
Most Helpful Guy
Let's put ISIS as an example: If ISIS doesn't have to protect their land, they don't need to have many followers there, therefore they can send more "troops" to attack other countries. If the other countries attack, ISIS won't send troops cause they have to defend their territory. Also, there more "bad guys" you kill, the less chances they have to attack your country.
Another example is Hannibal's situation. He wanted to attack Rome, so he took a long journey to get them by surprise, but the Romans attacked his city, therefore he had to go back to protect the city. Rome's attack stopped the advance of Hannibal and his troops, and the same thing applies to the US (and other countries too, it's not just the US who has troops in other countries fighting wars).2