I think that's Melilotus officinalis, which was introduced to the US as a forage crop. The common name is sweet clover because the leaves contain coumarin, which smells like vanilla.
Thank you! That does seem to be it. It popped up this year and I've been letting it grow to see what it becomes. It's actually kinda pretty and I'm thinking about encouraging it around the perimeter of our house. The dragonflies seem to like it.
