Dog acting out

Question
My dog is 2 1/2 years old and he's a Beagle. I've had him since he was a puppy. He started off as quite a problem as he was quite stubborn, but he became and is a really good and well behaved dog. His one real problem is he has a tendency to only listen to me. My ex-girlfriend always complained that she was the "alpha-male" because she had to discipline him the most. I told her that I never had to discipline him because he listend to me. His name is Basil and he's a full blooded AKC registered Beagle. He and I are pretty inseperable and have a pretty close bond, I'd like to think.
  We moved alot in the past year and he's been around all kinds of people and dogs. He's played with little kids before and he was never mean.
  My ex and I moved to Vegas and lived with an ex professor of mine who owned an old golden retreiver and he was wonderful with her, playing, etc. She couldn't move much and  Basil adapated to playing with her on her terms, never biting too hard.

Then we lived with some poeple that had two large outside dogs that were grouchy and mean at times, yet he got along with them also as long as he stayed away from their food.
  My ex and I split up and my friend and his wife took Basil and me in.
  We currently live with three female Beagles who aren't as well behaved because my roomates aren't much on discipline. He gets along fine with them though.
  I've noticed that he's been taken up some of their qualities, i.e. eating dog feces outside, begging, etc. I try to enforce the same standards on him that I used too but as the other dogs aren't mine, I think thats a problem.
  Anyways, last month I had to go out of town and he was left with my roomates. Apparently, he got incredibly moody while I was away and at one point when he went after another dogs food, my roomate barely smacked him on the butt and told him no, my dog snapped at him, that was the only time that happened to my roomate.
  He's snapped before but its been varied and awhile back.
  I was gone a month and came back and my dog has been a little unwilling to leave my side but aside from that he's seemed normal.
  Today, I was watching him outside and saw him eating some poop, so I went out there and told him no, and smacked him on the butt. He's old enough to where I don't have to hit him hard to make an impression, just tap him and he knows thats bad.
  When I smacked him, he turned and bit at me...when I withdrew my hand he jumped up and attempted to bite it again, and then turned up to me and was snarling at me. I've never seen my dog snarl in his life, he's always been of good temper.
  When I reached for him again he took off running around the yard. When I finally caught up to him and grabbed him by the scruff of the neck, he started turning on me and trying to bite me. I had to hold him to the ground in order for him to stop.
  When I let go, he turned and just snarled at me again.
  After sitting there for a minute and starring him down he stopped and looked away and refused to look at me. Then he allowed me to pick him up, I took him inside and I now have him in a cage.
 I really don't know why he's started snapping at people as its never been a problem in the past. This was way more then I've ever seen.
  Do you have any suggestions?

Answer
Hello Jack, Your poor dog has had quite an upset in his pack leadership. Personally, I do not believe in correcting a dog with a hand, as it will cause problems in the future. Toss a stick at him, a ball, or make a loud noise to get his attention, then discipline him in a firm tone. He needs to know that you are still his fearless pack leader and will protect him from the other dogs and the other people in his life. What you experienced was fear aggression, which is common among dog packs. Take him for as many walks a day as you can at the heel, to help reestablish the pack leadership. Do not let anyone else in the home discipline your dog besides saying no to him. You did the correct thing by holding him down to the ground, looking him in the eyes until he turned away. A kennel cage should never be a place for punishment, but a safety zone, a den of sorts. He went through some hard times while you were gone. Aside from working with him to establish yourself as his protector and pack leader, the only other thing I would recommend is to take him to a trainer, behaviorist, or kennel that will watch him for you and treat him fairly while you are away. Please leave me some follow-up and let me know how you are progressing. Thank you for writing. Regards, Susan