Hard to tell from the picture, but check out Houttynia cordata. Looks similar, but I may be wrong. If my guess is correct allow it to grow, use herbicide - then allow it to re-grow, use herbicide - keep repeating. A plant that should not be allowed. hortster

Thank you! Wow, this everywhere on the United States map. Surprised I have never seen it before in my yard.

Almost certainly Galium aparine, aka cleavers or bedstraw. It pulls out easily in moist soil but also breaks off easily. My pup is forever bringing in pieces of it attached to his fur after doing his rounds of the exterior of the garden where it abuts the greenbelt.

You can still hand-pull that, but if it's spottily in other parts of the lawn as well, I might suggest spot-spraying with Weed B Gon Crabgrass Killer. Mix up the concentrate in a one-gallon sprayer (it's cheaper that way) and keep after it to avoid having the CG seed out.