These popped up in my backyard this summer underneath and near my redbud tree where there is no grass. When I first saw them I thought they were Pride of Barbados (I have some nearby), but then I saw that the leaf structure is different. They seem to stay somewhat short; however, I do mow them when I mow the grass, so I'm not sure how tall they would get otherwise. They have the spores under the leaves like a fern.
Wow, thanks so much for the replies. I had no earthly idea where to start looking. I've searched all my native Texas plant books and came up empty. I was afraid it was a non-native that I would have to fight. This one was not in a book that I have. However, after doing some online reading, it is definitely a legume and probably indigenous to the prairies around where I live. I suspect the birds may have brought it in. I rather like it, and it is beginning to make a very nice groundcover where grass won't grow.
Weeds can be friends, in the right places! Any legume enriches the soil, so this plant might be a "pioneer" weed that replenishes the soil and makes it fertile for new plants to move in someday in the future.