Why is my older dog now acting aggressive at the dog park?

We have a 2 year old female boxer and now a 5 month old female boxer. We used to bring our older one to the park all the time before the puppy and she was great, never got in a fight. Today there was only one dog in the park while I was walking them both so i figured it would be a good time to introduce the puppy to the park. As soon as we got in the other dog snapped at my older one and they left. Shortly after another dog, about 8 months old, came in and my older one snapped at her. I had to pull her off and hold on to her while the other dog played with my puppy, who was loving the park. I could only stay a total of 5 min because any time the other dog came near my older one she would snap and other dogs started showing up.Is our older dog being over protective of the puppy? Or was it a doomed attempt because as soon as we got there the first dog snapped at her? Any suggestions on what to do? We want to be able to bring them to the park to socialize and I have always had a good experience but not i'm just worried about what would happen.

It's quite possible and likely that she is protecting the puppy. Hopefully, you'll be able to get through to her that the puppy is fine. I know it would be hard, but you may have to consider bringing them separately.

with large breeds particularly it helps if they are all medium natured, none of them up and alpha.so if your boxer wants to be the boss and other large dogs have the same attitudes then there is a problem. and the way dogs work this out is by either leaving or becoming submissive or standing ground... obviously if dogs are smaller breeds this isnt so much of a problem. most people know the potentials of their dogs and keep them away from these places if problems could occur.if you dont know the natures of the dogs using the facility and one of yours will "fight back" or not back off on confrontation then you might witness a dog fight...if you ask around you might hear of such problems.

You had two things happening at once so its difficult to know which if any caused the problem.Take both of them again and see how things go.