Weed identification and removal
You've got blackberries, specifically Himalayan Blackberry, Rubus armeniacus. These are invasive in our area rather than a native plant, though they are naturalized into nearly every landscape in Washington County. Removal will be a multiple-year project but can be successful. There are mechanical and chemical methods or use a combination of them. The time of the year matters, so follow the guidelines in these publications.
Methods to control blackberry thickets: https://extension.oregonstate.edu/node/81706
Blackberries: Friend or Foe?: https://extension.oregonstate.edu/pests-diseases/weeds/blackberries-friend-or-foe
Himalayan Blackberry (Tualatin Soil and Water Conservation District) : https://www.swcd.net/invasive-noxious-weeds/himalayan-blackberry/