Yes drunk driving is illegal in all 50 USA States and it's a good thing it is even though it doesn't stop everybody from driving drunk.
Driving and being drunk while driving is a bad thing and causes fatal wrecks and the people who usually die as a result of the wreck with a drunk driver is the person who is not drinking.
Having a beer and then driving later is not a problem but when you become drunk you're really impaired and cannot make good decisions and then you get into a serious crash.
If you're drunk and kill someone while driving drunk then you can get more prison time than just a DUI.