tree of mystery

Now that the bird shed, etc. is gone we're starting to make the backyard our own.  Except that it's a slow going process {like everything with this house} and requires battling some shoulder high weeds.  We'll need to grade the yard so that it stops sloping into the house and garage.  Build back the deck and put in some raise gardening beds too.  Oh... we've got plans!

While that's all {slowly} going on, I've got to put it out to the internet to see if anyone can help us identify the tree that's {growing?} in our yard.  I'm pretty certain that when we first saw the place back in October this small tree had leaves on it.  But by the time we moved in at the end of December it was looking much more like a bundle of sticks with some nasty thorns.  Then it rained {yay!} and leaves started popping out from most of the sticks.

We're not quite sure what to make of this little tree and aren't sure what kind it is.  So, how's your tree identifying skills?  Below are a few more pictures and I'd be happy to answer any questions - especially if they get us closer to understanding what we've got.  So, anybody??