Where do you live?

If you live in a marshy area I would say this is Brazilian pepper brought in by birds. Looking at the sand on the roots, I'm leaning toward the pepper. That plant is listed as a noxious and invasive species in several states along the Gulf Coast. It is also listed as an attractive and desirable ornamental tree in other states, so I thought I'd ask. If pulling it out is fairly easy, you could do that. Birds are
relentless so this will be a never (ever) ending job for you. You
should be able to spray it out with an atrazine product, but those
products are severely limited in the frequency you can spray. This is
for good reason, so don't spray more than twice a year.

If you live in a dry area then it might be mesquite. Mesquite is almost never a desirable plant. At least with mesquite you should be able to pull it all out and not have it return anytime soon in large numbers.

Looks like Chamber Bitter to me.

I live on Hilton Head Island, SC. By the looks of it, it's this chamberbitter. Now I just need to get it under control. Thanks