What is this and how do I keep it from coming back
This is either our native sumac, or more likely, the similar but very invasive Tree of Heaven (Ailanthus altissima).
Here is our page on Tree of Heaven: https://extension.umd.edu/hgic/tree-heaven-ailanthus
and a good way to tell them apart on the fact sheet from MD. State Highway Admin:https://www.roads.maryland.gov/OED/Tree_Shrubs_Fact_Sheets.pdf
Once it has leaves, crush one and smell. Tree of Heaven smells like rotten peanut butter. Cutting them down seems like an invitation to them to grow back even stronger.
For one growing up within a shrub like azaleas, you could cut the trunk, and then immediately use a sponge paintbrush to apply an herbicide containing glyphosate or triclopyr to the cut trunk, being careful not to contact the azalea.
Thank you very much. It is probably the Tree of Heaven (should be the other way around but I digress) because I cut one down before and didn't place any killer on it and it regrew shoots out of the stumps.
You're welcome. It is a tough one! Good luck and don't hesitate to contact us again if you have additional questions.