Aggression in dogs must be addressed as soon as it is first observed and from then on every time thereafter. It is NOT something to ignore or handle later.

Here is a fabulous article on the subject. It describes the types of aggression and then the way it can be handled.

But if you handle your pup properly, aggression should never become an issue.

My main advise: no physical punishment of any kind. NO forcing it to do anything it doesn't want to do. As soon as you see it is scared or stubborn about something - get out the treats and  train it to accept the situation or to perform the behaviour. It probably just didn't understand a situation or thought you could be tricked into taking a subservient role. Make it fun, but also make sure you are the one who takes charge. The pup needs to know you are the leader. 

Here is the article:!.htm

If you see a problem in your pup's behaviour - read the article please! Then get some help if you need it.