I'm trying to ID a vine I've been fighting for years. If I know my enemy, I can conquer.

It's a very thorny vine, and does not branch. It usually starts beneath existing bushes and trees (Forsythia and Cedar) and grows to the top - even higher if it can find a taller neighbor. The thorns are 1/2" to 3/4" long, have no discernible growing pattern, and can easily penetrate thick leather gloves. I've attached some pics of a small piece I cut out today.

It looks like Bristly Greenbrier/Smilax tamnoides. Here is a link with photos:
http://www.illinoiswildflowers...

Others may have a more accurate answer.

Edited to add:
Whichever kind of Smilax/Greenbrier it is, cutting the vine will not eradicate the plant. You will need to dig, dig, and dig some more. Get a crow bar, pry bar, pick ax or whatever tool it takes to remove the huge root mass. Use a Google Image search with the keyword 'greenbrier tubers' to see what is underground.

Edited to add link.

Moved to the Plant ID forum so it can be marked as 'solved' when you are satisfied with the ID.

Saw Greenbrier (Smilax bona-nox)
Yours looks like a different species (there are quite a few species of Smilax) but there are some tuber pics there showing what they look like at the surface level, and some pics of excavated ones.

Maybe Roundleaf Greenbriar- Smilax rotundifolia, the leaves seem to match but not the amount of thorns along the stems..
http://marbleheadconservancy.o...