Answer: Unfortunately, based on your description I am not certain what plant you have. Here is a photo of a variegated, annual euophorbia that grows from seed; it has a milky sap that can cause dermatitis which is a good identifying characteristic. You may need to cut and paste the complete url into your browser to make it work correctly.
Here is a photo of a phlox that shows the typical flower shape and foliage and stem quite well, although this one is pink.
It might also be the notorious garlic mustard (Alliaria petiolata, A. officinalis) which smells like garlic when crushed and is a biennial, although it is more of a spring bloomer. To stop it from spreading, deadhead to prevent it from seeding.
If none of these look correct, you might take a photo, both close up of the bloom and an overall shot of the plant and consult with your local professionally trained nurseryman and/or your county extension. Since you are not certain what it is, do not let it set seed in case it could spread invasively. I'm sorry I can't be more specific for you.
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