For the future - I recommend posting you location in your profile and also taking better pictures from different angles - including the stems and how the leaves come off.
This one is difficult for me.
My initial thought when I looked at the plant was one of the Galinsoga Species that hasn't flowered yet. These are commonly referred to as quickweeds, smallflowers and a few other names. One of the more well known plants in that family is Gallant Soldier.
Without more info and better pics, that is a first wild guess, and I'm not sure I could ID it if I had that.
Also, with your description of how it took over, it may be some invasive that I don't even know about. But, if that's the case the county agent in your area probably has been given information.