The Nemesis Tree

Okay, so I'm fighting this invasive species of tree at the home compound. Every time I cut one down, half a dozen others sprout up. The stupid things have thorns and resist every technique I can think of to get rid of them. I'm no rocket surgeon, and one of the many things I don't know about is trees. Even though we have a national park inside the compound.
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Based on internet research, I originally thought it was a Russian Olive tree. All the descriptions fit my plant nemesis until I noticed that my trees did not have....olives.

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I then, with complete confidence, decided it was the Tree of Heaven. All of the descriptions, especially those describing its hardiness and aggressive growth, fit. Unfortunately, once I really looked, the leaves did not match.

So I have an unknown species of aggressive, intrusive tree on my property that defies identification and obliteration. Any arborolgists out there?

Leaf comparison:
The Nemesis Tree:

[7/9/08 Update: We're hiring landscapers to come out and work on the grounds of the compound. They have no idea what the Nemesis tree is, and moreover, they've never seen anything like it before. No one else in our town has anything like it. We may be making first contact with an alien race -- if we all disappear, contact the government. On the other hand, we may be eradicating the last survivors of another planet.]