Can you cut a tree in half using a hydraulic mechanical arm and a very sharp steel sword in one swipe

A Chainsaw takes too long to cut a tree.

Sort of. What works better is more like jumbo-size hydraulic pruning shears.

There are companies that make attachments for skid loaders, excavators, and such.
http://www.youtube.com/watch?v=srq8q8AKj…

what's the diameter of the tree?

1 inch?
1 foot?
1 yard?

kind'a makes a difference don't ya think?

(anything much bigger than 1 inch, probably not.)

OH, and is it balsa? pine? oak? That'd make a difference as well.

I doubt it, depends on how dense and thick the trunk is