Can any one tell me why US should stop Global Policing?

I have to think about 3 reasons why US should step out from the Global Policing...

I have got the first 2, but I have no idea about the 3rd one, can anyone give me a hand?

1. Sponge over US global image

2.Decrease arms and war expenditure

Then I don't know what to do with the 3rd one.

Thanks~~!