Hello. Based on your photos that looks to be myrtle spurge, a Euphorbia.

The profile for this plant is found on the Oregon Noxious Weeds website: https://www.oregon.gov/oda/shared/Documents/Publications/Weeds/MyrtleSpurgeProfile.pdf

This profile suggests a biological control, " a leafy spurge flea beetle", but otherwise the chemical controls noted in the factsheet below might be more effective. It states that any control program must be long-term. Be sure to note the "cautions" so that other plants or animals are not damaged by chemical treatments.

The PNW Weed Management Handbook has a factsheet for managing a variety of spurge plants: https://pnwhandbooks.org/node/8566/print

The City of Boulder has perhaps the best response: https://bouldercolorado.gov/ipm/noxious-weeds-myrtle-spurge "On your property, Myrtle Spurge is easily removed by hand. Caution: Wear gloves, long pants, and a long-sleeved shirt when pulling. The weed exudes a milky sap that can severely irritate skin and eyes and is toxic if ingested."..."Remove at least four inches of the root when pulling myrtle spurge. The weed has a taproot, so the more of this you can pull out, the more effective it will be at preventing re-growth." Remove plants prior to seed set. Finally a good factsheet:https://www.colorado.gov/agconservation/Factsheets/MyrtleSpurgeFactSheet.pdf

Best of luck controlling this plant. It probably is well-suited to your site, but not a family friendly plant.