All the US intervention are based on pure self interest, nothing to do with policing the world. If anything most of the world would have been better off without US interventions, particularly latin america and middle east!
Now let's say you don't have conventional weapons.
China invades all South East Asia progressively. Would you nuke them? Russia invades Eastern Europe. Would you nuke them? If yes why hasn't the US nuked Russia yet?
Then all your allies suddenly switch side after being overthrown by dictators who are in favor of your enemies. Would you nuke them?
Another one: you are terrorists associations preparing attacks against you in camps all across middle east. Who do you nuke to defend yourself?
Conventional weapons are actually the only useful tools nowadays, because war is much more sneaky than it used to be, and it is much more about fighting for influence than really invading countries or defending you territory.